Sounds like standard bullying tactics - provoke the victim to the point where they throw the first punch. The victim is the one who gets expelled for fighting and the bully gets their status quo back.
The two biggest reasons for getting bullied that spring to mind are
1) He's cleverer than the other kids and makes it known in class*, or makes them feel small in ways that he maybe doesn't realise
2) He's to meek and allows himself to be picked on. Newcomers aren't allowed into the social group ("gang mentality") until they show signs that they can stand up for themselves ("handy in a fight")... but not so much fight in them that they threaten the leader(s).
* he might not be able to avoid doing this, if a teacher insists on focussing questions on the new kid, to test them out and he just happens to be capable of answering correctly. So I don't mean he's showing off just that he's being quizzed and performing well. If other kids mistake this for being "teacher's pet" then you know what comes next.