The articles suggested here by dgraab are very helpful in understanding the reasons why students become or resort to bullying.
Basically, bullying is a cultural phenomenon within school populations that may have various reasons, including depression and anxiety in the aggressor, an attempt by the aggressor to achieve power over the victim, and/or a behavior that is stabilized by peer "accessories" who stand by without assisting the victim.  Often the peer observers laugh and actually in some cases, popularize the aggressor as "cool."  In some cases, particularly when the bully is a high academic achiever, teachers and staff may not even recognize the bullying behavior.
Whatever the case, bullying appears to be a power play by the aggressor. Unfortunately, in too many cases, it succeeds.
Do check out these articles.  They are quite informative.

Bette J. Freedson, LICSW, LCSW, CGP