Why and how did bands like RKL, Ill Repute, Agression, Stalag 13, and Dr. Know come up and be so freaking awesome?
Big generalisation: great music tends to come out of two sorts of places. Either places where music is accepted, and thrives, and the place encourages it : Austin, Nashville, wherever. Or the opposite, where the edginess or boredom of the place drive people to make great music as a way out : Woking, UK, subject of the Jam's "Town called Malice", Coventry, UK subject of the Specials' "Ghost Town". Looked at that way, Oxnard seems to be the second kind of place.
But also where a cluster of bands come out of the same place at the same time, there has to have been someone to come first, and make some music that caught how people were feeling at the time. Without that, nothing would ever happen. Not sure who came first, with Oxnard or even if it's possible to pin that down. And someone to encourage that - Mystic Records, I guess. I'm basing that part of my answer on independent record labels like Factory records or Rough Trade records, and the various bands and musical scenes they encouraged.
This answer is a view from a distance. An insider's view would also put it down to quirkier local things, I'm sure.