Why do some of the abused become the abusers themselves?

Why do some of the abused become the abusers themselves? I have had time to ponder, and found this blog; http://www.amyreesanderson.com/blog/if-people-are-trying-to-bring-you-down/#.U8Em-fldW It makes me wonder why some abused people become the abusers themselves, but...