Circus factions

[Epictetus]

Would anyone know how / when / where / why the Circus factions became politicised? I have come across some quotes from Pliny the Younger and Marcus Aurelius damning the factions, but to me these seem unclear if it was on a sporting or political level. Tertullian is a bit wild-eyed and I don't know how much I should trust what he says.

A wild speculative guess: Caligula was a partisan of the Greens. Could the “opposition” have favoured another faction just to spite him and this started it?