why do people bully?

There is this girl that used to be my best friend then out of no where she just turned on me. I don't know what happened. Sometimes I wonder if her being friends with me was just being fake to begin with. We are both on the tennis team and she is spreading rumors about me and is trying to get everyone to hate me on the team. I cried so much about this. There is so much more things that she is doing to me. She always give me dirty looks on the court. And she got 2 other girls to really hate me. I just want to have a good season. She got most of the girls to vote for her for being caption and I told coach that she is harassing me. So she change the captain to me. Now she is extremely mad about that. But I honestly can't deal with her bullying me. There is so much more than what I said. I could write a novel on everything she ever did to me that was bullying but I'm not going to because I want to make this short. I honestly don't know what to do anymore. Please give me advice. It could be anything really, I just want to know what you girls think. Thank you