The French Chronicles, in particular, make out Edward of York to be the guilty betrayer of Richard II. This was certainly believed in France, because the Count of St.Pol hanged an effigy of Edward from a gallows outside the gates of Calais! English sources are a bit more circumspect. but the general impression is that Edward was not exactly Mr.Popularity of 1399/1400. So let's consider the first phase - Edward's conduct in relation to the Irish expedition.
It was probably not a good idea for Richard to hare off to Ireland at this time. There was nothing going on there that was an immediate threat, and it seems he was merely fired up by the breakdown of order in the country. He may have thought he'd settled things in his successful campaign of 1394/95. If so he was deluded.
He may also have doubted Henry would be able to mount a successful invasion. This is not as crazy as it sounds, as Henry was in France, and France was supposed to be Richard's ally.
Edward is criticised for being late to arrive for the muster. He had a good excuse. Richard had sent him up to the Scottish Border to settle some details of the truce. The Scottish Border was a long way, there and back, and presumably he then had to put his retinue in order. It's possible, even likely, that while he was up there he gained some idea of how disgruntled the Percy and Neville clans were. We know he advised Richard to create another wardenship as a means of placating them. However there's nothing to indicate that he got into treasonable discussions.
Was he in correspondence with Bolingbroke? Again, we can't be sure, however it's perfectly possible. He was later to develop a strange love/hate relationship with Henry IV, but nothing Henry did in 1399 suggests that he thought of Edward as a supporter. On the contrary, he was swift to strip Edward of his most important offices, even before he became king.
Over in Ireland they were slow to hear of Henry's landing, and then unable to react because of contrary winds. It's at this point that Richard split his forces, allegedly on Edward's advice. They were in Dublin, and there weren't enough ships to embark the whole force. The decision was to put Lord Salisbury and part of the army into what ships were available, with the object of landing in North Wales and mobilising Chester.
The rest of the army marched to Waterford, and embarked there for South Wales.
Splitting the army was possibly not a good plan, but ships couldn't be created by magic out of thin air, and they were propelled by the wind, not steam, so they were to a large extent at the mercy of the elements. Assembling all the ships in one place would surely have delayed Richard's return even further.
In short, the decision may have been mistaken, but even if it was made on the basis of Edward's advice, that advice was not necessarily treasonable.