The NBA is a lot more personal between teams than most of the other major sports in America. Everybody knows each other and they play each other three or four times a year, making each meeting special, but not too usual.
Instead of being an emotion-fueled single meeting a year like in football, or a passive competition between clubs over a marathon of a season like in baseball, there's more of a hockey mentality. Players are right up next to each other for the duration of games, and they end up pushing and shoving for 48 minutes a game.
Obviously, when you get personalities like this on the court so often, something's going to snap eventually. There aren't guys beating each other up on a nightly basis, but they do tend to throw down every once in a while, and it's usually surrounding the same core group of guys.
Some guys just have an intensity about them combined with an attitude that is not only captivating, but full of anger to a point that they just end up being rougher than other players. These are the hotheads who keep the league interesting past the basketball aspect.