Why do peoply bully one person and a nice to everyone else?

Either that victim is causing them to bully him/her, i.e. The bullies just desided they were annoying, too diffrent, or the bullies were insecure or jealous.

OR ,because theyre bullies no matter what, they have personality issues (no excuse) and dont have self control.

Dont let them bother you, especially since no one deserves being bullied. No one.