Why do guys gets so mean when a girl helps them out?

This has been bugging me for a long time so I am gonna get it off my chest. So as I was walking outside toward the library at my school and I just so happen to notice that these guys were bullying this guy, roughening him up a bit or something who knows, it looked like they were messing him, they were pushing at him and stuff, so he looked like he needed help, I swear he did. So I go over and try to help them, telling the guys that I would bring a teacher over if they didn't leave him alone. So after they left, the guy rudely said he didn't need my helped then left. What the heck is with that, I helped him, I did nothing wrong and I've been worrying about this so much that it woke me up, I just don't get what's with this guy. If it were me in his place, I would want to all the help I could get.