In my opinion, there are many reason. The way they look, act, talk. The list is long. I believe it is insecurity in the child that is the bully. They hurt others to make themselves feel better. I try not to focus on the reason, but how to properly raise my son so he is very secure with himself. And to not care what others say or think. It's been a long process but he is starting 2nd grade and so far so good. He is not afraid to be him and to do silly things that others would never do. He got off the bus with these huge pipe cleaner glasses with things hanging everywhere off them. They were bigger than his head. (No joke) I even laughed when I seen him. I asked him what others kids thought about them and he said a lot laughed at him and some said he looked like a dork. But, it doesn't bother him because that's who he is. Weird and silly. Loves making people laugh. Confidence is the most important thing through life!
I hope your child is not being bullied :(