Here's my theory on Diggs. Most of it is simply that he is a selfish and petulant person. That's the majority of what happened. That being said:
During the 2022 season, I think it started with the Hamlin caridac arrest game. If you remember, Diggs was trying to rally everyone on the sideline to go back out there. When that was ultimately overruled by other players (likely led by Allen) he probably felt like his input was ignored, and he probably felt like the team "quit."
So now he had it in his head that he has no say over Allen, and when they laid an egg against Cincinnati and Allen played poorly, the BS just started snowballing. "I'm the only one who really cares about winning; I'm just as important as Josh" and so on and so forth.