This article attributes the increasing incidents to a 1998 episode of the cartoon South Park which featured the behavior apparently, as well as the proliferation of 'viral videos' on You Tube in recent years showing these attacks.
I'm a little skeptical about multiple aspects of this assertion, however. This was already huge when I was in elementary school in the 1970s. When I was in 4th grade, almost every guy in my class had to walk around the playground during recess in a perpetual protective crouch, ever vigilant against attack. Woe betide any kid who momentarily let up his guard. He would be on the ground in a fetal position within seconds. And just as suddenly everyone would then be standing over him laughing, Lord of the Flies-style. Mercifully for all, this had all ebbed by 5th grade, when girls became more interesting than punching other guys in the shorts. Though I must admit that, in spite of myself, the modern nicknames for this behavior that are cited in this article did make me smile.