Did you miss the fist fight in the locker during the Miami series? Rumor was it was Smart going Tatum and Jaylen stepping in. I've read that Smart and Tatum aren't exactly best friends other places too. Irving wanted out. So did Horford. Now Gordon. You believe what you want. I choosing to look at the evidence.
A fist fight after a tough loss isn't a bad sign to me. Especially since they came out and fought hard to win the game following that. The players are super competitive and they care about winning. Sometimes that's going to result in conflict. Brothers fight all the time, it doesn't mean they hate one another.
Irving left because he was a weirdo and he poisoned the well here. I can look at the evidence and see that he was pretty clearly the problem.
Horford left because it appeared the team was not going to be contending, and he got a massive offer from another team. He has since expressed that if he knew the team would be getting Kemba that he would have tried to stay.
Gordon is likely leaving because he can get better money elsewhere and wants a larger role than the Celts can offer him.
I don't think the Celts are perfect, but I think they're far from having a "suspect locker room."