It started with Hayward and the pre-season, when Stevens seemed more interested in who was going to make the cut of the last few players than what the starting rotation was going to be. He never bothered playing the regulars much in pre-season games, and then suddenly the starting rotation for the first season game was the money! He should have known thay Hayward wasn't 100%, and he had no business being in the starting rotation in the beginning. I think that caused some resentment of the young players who had played so well down the stretch. Kyrie Irving wasn't 100% at the beginning of the season, either, though it wasn't the same as Hayward and Rozier's resentment was less founded, in my view. Rozier was the backup. That was his role.
I blame both Ainge and Stevens on the Hayward misjudgement, since both share in personnel decisions.