My bystander experience

I don't really know but I would like to know why do people think it is okay to bully. I once saw this little kid in school and some kids were teasing him because he was short and he was being pushed and tossed around . I remember I was mad because they looked older than the little kid and just looked so sad. I yelled "Hey, Don't you have something better to do ?"I was older than the bullies but they still thought they were tough enough to answer back. So one of them tried telling me  "Yeah and you care why ." I was already mad so I pushed off all the bullies off the kid and told them to go home. The little kid thanked me and told him don't worry your welcome and walked away .