Why is Rowdy so angry? What is he fighting?

The appropriately-named Rowdy is a pretty combustible character; he's liable to catch fire with rage at the drop of a hat. He's by far and away the toughest kid on the reservation and forever getting into fights. Rowdy is angry, alright, but to be fair, he has an awful lot to be angry about. His father is a nasty, abusive, violent alcoholic who'd beat up his wife and son as soon as he looks at them. Rowdy's extensive experience of domestic violence teaches him that the only way to get any respect in this world is if people fear you; and people will fear you if you're violent towards them.
Rowdy is not a bad kid, on the whole. He's a good friend to Junior and always looks out for him. But even with his best friend, Rowdy is never far away from using his fists, as we see when he gets mad with Junior for changing schools. Rowdy lashes out at Junior because he fears he's losing control. Once again, this is rooted in his troubled home life, where he has no control at all and where he's at the mercy of his bullying drunk of a father.