Why do bullies like to even bully?

I'm a 15 year old guy and im not a nerd or anything (im not concieted) but im a cool person wit lots of friends. My friends left for a few mins to get something for us eat when i was at the mall in the food court. So i was sittin by myself and these random group of kids come sitting next to me. They were all little kids accept for one of them. The little kidz were like the bullies lil minions and the bully was probably like 17-19. He slapped my head and I hit him back HARD and he stepped up to me and made comments about me that werent true to make the "minions" laugh at me then left. I wasn't scared or anything but idk what his problem was. Then when he left I saw him bully two little kids and threw them to the floor and kicked them. Again... makin the "minions" laugh. Why do bullies like 2 do this? And why in groups? Are they jealous of people like myself or what!?