Prison gangs formed from a kind of arms race of mutual self-defense.

Lots of gangs form that way -- it's one of the two main pathways to organized crime, the other one being economies of scale in selling illegal goods and services. The Bloods, for example, started out as a sort of anti-Crips self-defense force, and many yakuza organizations are generally thought to have their roots in mutual-protection societies among small commercial enterprises.