Girls have always fought. When I was in high school, it was more common to see two girls going after it than it was for the guys. I think it's just the weaponry that's changed - we used to just shove or maybe hit each other, now they whip out the knives and guns.
But it's like that with boys, too - the escalation of violence. You can't sit around all day with your pop culture telling to to bust a cap in someone's ass and not see a rise in violence.