My guess is Brown challenged Kyrie on and off the floor. Called BS on a lot of what Kyrie said about the team as well as real world issues.
Kyrie got mad and took his ball and went home.
What I find most interesting is that Marcus Smart publicly backed Kyrie at the time Kyrie and Brown were having a public spat. Tells me all I need to know about who was right (Kyrie) and who was being a baby (Brown).
I think Marcus was just trying to hold everything together at that point.
Bingo
Just my guess from personal observation...
Clearly Smart is intense, a guy that gives 100% all the time. Brown plays hard too, but often over thinks things,
or falls asleep, etc.
Everybody makes mistakes, both of those guys are far from perfect. When Smart makes a mistake, you can hear/see him say my bad, slap his own chest, take the blame.
When Brown makes a mistake, I don't think he always owns it (and personally I think that's what got him in Stevens dog house a lot). I've seen other players yell at Brown, or at least be visibly upset with him, and I think a big reason why is he doesn't seem to own his mistakes.
This play for example, Brown's man blows by him and scores, and Horford is upset going into the timeout. If that happend to Smart or Morris, they're more likely to slap their chest right away and say "my bad, that's on me." When Brown makes mistakes, he looks lost or just puts his head down. I can see that really rubbing a guy like Smart the wrong way. It's okay to mess up, just own it, don't act confused about what just happened! Personality clash if anything.