Prison violence is a sort of tit for tat game. The southern mexicans have the numbers and organisation, but thats not gonna stop the other groups from defending themselves.
It wasn't an act of delf-defense. Initial reports is that they instigated the fight.
The guy in the video said that the two groups were having meetings before the assault. Relations broke down and violence was inevitable, any action against a group the size of the surenos by a much smaller inmate population such as the asians is self defence. Different rules apply in prison im afraid.