Jim Goad has a good history of Knockout Games, noting how far back it goes (“Hunting the Domestic Polar Bear“):
Robbery is hardly ever a motive. It appears to be done for fun, to establish ingroup status, and to send a clear message about who rules the streets. Throughout history, those who rule the streets have often served as the shock troops and enforcers for those who rule the nation. This isn’t some empty nihilistic ritual—it’s a highly political act of establishing dominance and marking one’s territory.