I think most 'normal' people (and we'll use that term loosely) want to fit in, be part of the group, the crowd, particularly the cool group, or whatever other term you want to use for it. Part of the pack, as someone else said. And the pack will single out anyone it considers different, anyone that doesn't conform to it's social standard. I think there is also sort of belief that if they join in on any bullying, then they won't get singled out themselves. Protection in bullying. But for the most part, I think bullying is about conformity and power. Bullies like the feeling of power. They seem to like the feeling of hurting someone, physically and/or mentally. They like destroying someone's life. It makes them feel powerful.
I was bullied before I started school. I was bullied in primary school, high school, and university. I was bullied at work, by people of all ages, some well in to their 40's and 50's. From my experience, bullies don't ever really seem to mature. If they encounter someone who is singled out and bullied, they'll join in as gleefully as they did when they were 15, or even 5.