I think the real issue is Brad failing all season to deal with this divide between the vets and the young players who want/need more playing time. In hindsight Gordon coming back really effed up this team in terms of the balance and the distribution of minutes and touches. Then the demotions (of Gordon himself, of Jaylen and of Terry) must have been and must still be tough to deal with.
It's probably easiest for Terry to deal with it given Kyrie was clearly going to come back no matter what, but it must be difficult for Jaylen to get demoted for Smart and/or Mook, who were both on the bench last season, while Tatum gets more minutes and more usage, meanwhile losing minutes to Gordon who couldn't hit the side of a barn door the first month. And even though realistically Gordon probably knew he was far from his best at the time he got demoted, it must weigh on his shoulders a lot, the fact that he's on the bench, struggling, and costing the team $35m a year and costing his young teammates shots and minutes they had last season.
I think all that disrupted the team's chemistry, the togetherness, the trust. It comes out now and then but it's very brittle - a couple of losses and it looks like it falls away significantly. It disappears as soon as they are put under the pump by the opposition. First they miss a few shots, then it affects their defense, and then the whole thing compounds. How many times this season has this team gone behind by 20+? Then all the finger pointing. Think of how often the team's chemistry, togetherness, playing for each other, owning each person's shot as if it was their own, not showing resentment at their minutes, etc. has been brought up by players, I think the togetherness only lasts as long as they are winning. Really good teams, the togetherness, the effort, is greater in times of adversity.
It's Brad's job to prepare the team that he has, to play each game with the same level of effort, and to hold players who aren't putting in that effort accountable. But Ainge is sitting tight because he doesn't want to lose his assets for an AD trade, so he's prepared to sacrifice this season if needed, which probably limits Brad's ability to do that.