Bullies in Middle Grade Fiction

Bullies, bullies, bullies. I have bullies on the brain right now. Middle grade fiction often includes a bully antagonist. It's interesting watching my kids, their reactions to various bullying circumstances-- as a writer I glean useful tidbits from their experiences. What is it that leads some kids to bully? I remember when my daughter was about three, I took her to a beautiful park in Ojai, California. I sat on a bench under the oak trees, while she raced around the jungle gym. There were only about half a dozen kids on the play structure, plenty of room for everyone. A little kid walked up to my darling little girl and pushed her to the ground--for no reason--no words were exchanged--no toys were threatened. Of course I was outraged, I swept my daughter up from the ground and barked at the little monster. But what is it that makes children behave that way? Is it some innate survival of the fittest thing that they have going on there? What is it that makes some kids so mean? Many of my characters are bullied, probably because I was bullied as a kid, but in terms of understanding the bully? I'm at a loss. So what happens when the bully is an adult? How do you write your protagonist out of that situation?