I've already written several posts about Edward II's successful campaign against the Contrariants in 1321/22, and the executions of twenty or twenty-two men which followed. In addition, a few dozen men were imprisoned - various chroniclers give the numbers as sixty-two, eighty-three or a hundred  - while others were released in and after 1322 after being forced to acknowledge that they owed large debts to the king in return for a pardon. The men imprisoned included Roger Mortimer of Wigmore and his uncle Roger Mortimer of Chirk; Maurice, Lord Berkeley and his son Thomas, Edward's future captor at Berkeley Castle; Edward's former favourite Sir Hugh Audley and his father Hugh Audley the elder. I have no idea what Edward II's long-term plans were regarding the imprisoned Contrariants; did he intend to keep them locked up forever, or did he think that one day he might be secure enough on his throne to release them? That day never came, and with the exception of a handful of escapees (Hugh Audley, Robert Walkfare, and, of course, Roger Mortimer), the Contrariants imprisoned in 1322 remained there until freed after Mortimer and Isabella's invasion in 1326.
Several chroniclers describe a plot to free several of the Contrariants from captivity in early 1323, and there are also references to the plot in official government sources and an extant letter of Edward II. Maurice, Lord Berkeley and Hugh Audley the Elder were imprisoned at Wallingford Castle in Berkshire - which had formerly belonged to Piers Gaveston - and, according to a long account of the plot in the Vita Edwardi Secundi , "a certain esquire who had long been in his [Berkeley's] service used often to visit him...it happened one day that this esquire with three or four companions entered the castle, by leave of the guard, and because his visit was customary it was in no way suspicious. The same night Maurice invited the constable to dine with him, and all the doorkeepers and watchmen in the castle as well. As they were dining the esquire and his companions suddenly rose and demanded the keys of the castle, threatening with death anyone who resisted." The constable handed over the keys, and the squire let in twenty or so others, intending "to warn certain friends and to get his lord Maurice away at cockcrow together with the other prisoners." Unluckily for them, a boy "living at the outer gate, realising that affairs in the inner ward were not as they should be, secretly slipped out, went to the mayor of the town, and at once reported that the castle was lost, and that many strangers had entered." Thus was the plan foiled, and the sheriff arrived in the morning and ordered the men inside to surrender. The Vita says that Edward II's half-brother Edmund of Woodstock, earl of Kent, and Hugh Despenser the Elder, earl of Winchester, happened to be in the area, and entered the castle, where they found "Maurice in custody as usual, and the rest in the chapel." Berkeley "stoutly maintained that he had plotted nothing to the prejudice of the lord king...". (A leaf of the Vita is unfortunately missing at this point, so the story remains unfinished.)
The Sempringham continuation of the Livere de Reis de Britanie has a totally different account of events : it says that on 11 January 1323 Wallingford Castle was captured by Maurice Berkeley's wife, and that they held the castle for about two weeks until Edward II's forces besieged it and they surrendered. The Livere says that they were aided by Sir John Maltravers, Sir Edmund de la Beche and Hodgkin de Wandon, who "chiefly held the castle, [and] were bound and brought before the king." Two interesting things here. John Maltravers was another of Edward of Caernarfon's captors at Berkeley Castle in 1327, and is usually assumed to have fled from England after the battle of Boroughbridge in March 1322, though it's not impossible that he was still there or had returned temporarily and was trying to create as much trouble for Edward II as possible (his father John Maltravers the Elder was still in England and at liberty, and attacked a fair in Dorset in or shortly before October 1325).  Secondly, Maurice Berkeley's wife - his second wife, whom he married in 1316 or thereabouts when she was already in her mid-fifties - was Isabel de Clare, the much older half-sister of Eleanor de Clare and thus Hugh Despenser the Younger's sister-in-law. (Much as I'd love to think of Isabel, the eldest child of Gilbert 'the Red', earl of Gloucester and then aged sixty, holding a castle with her husband against the king and some of his powerful nobles, I'm afraid I just don't think the annalist's account is correct.) The Brut chronicle contains a short account similar to the Vita's , naming Maurice Berkeley's squire as Roger Wauton, and saying that Sir Edmund Beche and Sir John 'Goleinton' were taken to Edward II at Pontefract after the castle's surrender and that Wauton was drawn and hanged at York. John 'Goleinton' has been identified as John Wilington, a Contrariant and adherent of John, Lord Giffard, and pardoned in November 1323; John's brother Henry and Giffard were executed in 1322. 
Edward II, at Stowe Park in Lincolnshire, heard of Maurice Berkeley and his friends' plot on 17 January - perhaps six days later, according to the date in the Sempringham continuation - when he wrote to the constable of Skipton Castle to see to the castle's safety and report to him, because of "some strange and diverse news we have heard" (par ascuns estranges et diverses novelles qe nous avons oy).  On the same date, he sent his household steward and the sheriffs of Oxfordshire, Berkshire, Buckinghamshire and Wiltshire to "besiege the castle of Walyngford and to arrest all rebels who have entered therein."  The steward of the royal household was Sir Richard Damory, elder brother of Edward's former favourite and nephew-in-law Sir Roger Damory, who had died in the Contrariant rebellion ten months previously. (Natalie Fryde's Tyranny and Fall of Edward II, p. 261 note 26, misses Edward's letter and the order to Richard Damory and says that the king heard of the plot on 20 January.) On 6 June 1323, Edward II paid seven pounds, six shillings and eight pence to Drogo Barentyn, sheriff of Oxfordshire and Berkshire, "which he expended by order of Edmund, earl of Kent, Hugh, earl of Winchester, and Richard Damory, steward of the king's household, whom the king appointed to take into his hands Walyngford castle, which was lately held against the king...". The money included payments to seven footmen from 27 January till 14 February and twenty-three footmen from 14 to 23 February, and for four men to bring one 'Thomas de Fencote, a prisoner' to the king, and a horse for him.  (I love this bit: "6s 8d. for the expenses of the said four men returning home for five days; and 20d. for the expenses of the horse returning for the same time.")
Edward II cannot have been a happy man in early 1323; the rebellion of Andrew Harclay, earl of Carlisle, came to light at the same time, and that of Robert Lewer had taken place only a few weeks before. Rightly or wrongly, the king suspected that Maurice Berkeley's attempt to escape from Wallingford was part of a wider plot to free other Contrariants from the Tower of London and Windsor Castle, a suspicion he gave voice to in November 1323, some months after Roger Mortimer's escape from the Tower on 1 August.  On 5 February 1323, Edward ordered a commission of oyer et terminer "touching the persons who seditiously entered the castle of Walyngford, co. Berks, wherein Maurice de Berkeleye and Hugh Daudele, the elder, and other prisoners were detained, and held it against the king; as the king now understands that the said Maurice and Hugh and the other prisoners consented thereto, and kept the castle against the king jointly with the said persons," a commission repeated on 7 April with the addition of Windsor and "other of the king's castles." 
Maurice Berkeley and the elder Hugh Audley both died in 1326, before the invasion of Roger Mortimer and Queen Isabella, as did Mortimer's uncle Roger Mortimer of Chirk. Berkeley was fifty-five, Mortimer of Chirk about seventy, Audley in his late fifties or early sixties. Given their ages, I don't think we need to look for a sinister explanation for their deaths, and there is nothing to suggest they weren't natural; none of the younger Contrariants imprisoned in the 1320s, who would have been more of a threat to Edward and the Despensers, died in captivity. The account of the plot given in the Vita, with Maurice Berkeley allowed frequent guests and permitted to wine and dine his captors, and Roger Mortimer's escape from the Tower later that year by putting poison in his captors' wine while he ate with them and the constable of the Tower, argues that the Contrariants' imprisonment was hardly onerous. The unsuccessful plot of 1323 should, however, be seen as part of the general unrest in England in 1323 and unhappiness at the king's incompetence and his and the Despensers' greed and tyranny.
1) G.L. Haskins, 'A Chronicle of the Civil Wars of Edward II', Speculum, 14 (1939), p. 83; The Chronicle of Lanercost 1272-1346, ed. Herbert Maxwell, p. 62; Vita Edwardi Secundi Monachi Cuiusdam Malmesberiensis, ed. N. Denholm-Young, p. 100.
2) Vita Edwardi Secundi, pp. 129-131.
3) Le Livere de Reis de Britanie e le Livere de Reis de Engletere, ed. John Glover, p. 347.
4) Calendar of Patent Rolls 1324-1327, p. 232.
5) The Brut or the Chronicles of England, ed. F. W. D. Brie, vol. 1, p. 231.
6) Seymour Phillips, Edward II, p. 439 note 189. See also Roy Martin Haines, King Edward II, pp. 166-167; Natalie Fryde, The Tyranny and Fall of Edward II 1321-1326, pp. 155-156.
7) Phillips, Edward II, p. 438 note 187.
8) Cal Pat Rolls 1321-1324, p. 234.
9) Calendar of Close Rolls 1323-1327, pp. 656-657.
10) Foedera 1307-1327, pp. 537-538.
11) Cal Pat Rolls 1321-1324, pp. 257, 314.