This could be because they have low self esteem or that are not happy about themselves, and are just out to make themselves feel better. It could also be that they have experienced a disturbance of their own in the past and they basically don't know how to deal with the pain.
people bully because they are not proud of themselves. they need to bully others to make themselves feel more superior. it's like my school: the eight-graders are always teasing the seventh-graders, saying they are so much better; the seventh-graders get mad and bully the sixth-graders to make themselves feel good