Or defectors from the military. That was the case in both the Syrian Civil War, the breakup of Yugoslavia, and the first American Civil War - the nucleus of armed resistance formed around former military men who couldn't raise arms against their countrymen.

Private security forces and tech/AI are wildcards - we haven't had anything like the former since feudal mercenary days, and we haven't had anything like the latter ever - but could also form the basis for power centers.