As a child, I was bullied a lot, so I'm going to have quite a bit to say on this topic.
From what I've gathered, some bullies are so miserable that they have to pick on others to make themselves feel good. I was no exception to this. I was bullied and neglected so much by my peers, that I resorted to bullying myself, just to get myself noticed - by teachers, by students, anyone. I only had one friend that stuck with me through my bullying stage. They knew that it wasn't me, that I was just trying to get attention. Until one day I caught one of the kids I was bullying cutting in the bathroom. THAT, made me stop. So out of personal experience, I can say that some bullies are just doing it for the positive (or negative) attention that they get from their peers and teachers.
On a side note; Bullies, if you are reading this, which I doubt anyone on the MLP forums is a bully, please, reconsider your values and stop. You're ruining someones life by doing such horrible things to them.
If you are being bullied, please, do not hesitate to reach out to me, or an adult. I know it may be hard to do such a thing, but you have to, before it gets too serious. Your life could eventually be at stake, and I would hate to know that we lost a fellow brony to something as childish as bullying. So please, reach out, don't hide.