It can be for various reasons:
- Jealously. A lot of kids are teased for their intelligence or physical appearance. To be honest, a lot of bullies aren't the most attractive of people and very very rarely are they high performers at school, so they pick on others to make themselves feel better about themselves.
- Overconfidence. Conversely, some people think they're better than everyone else and will victimise those who struggle with certain things (especially disabled people or people with speech problems).
- Parenting influence. A lot of parents tell their kids to hit someone if they say bad things about you, or 'get them'. No child starts off as a bully, so their parents must influence them in some way.
- Peer pressure. We've all seen bullying in some form or another, and most of the time we do nothing or sometimes laugh along purely because we don't want to get on the bad side of a friend. I think this causes the 3 against 1 scenarios in particular, where one person is completely frightened of a certain group of people due to this.