Denver was originally divided into six "sectors," each controlled by a different nation that had signed the peace treaty that brought an end to the USA-AmerInd war, and shattered the UCAS.
When Ghostwalker strolled into town, he spent some time raining hell down on the different sectors, and then once he had everyone's attention, he ordered a cease-fire.
Aztlan chose to ignore the cease-fire, so Ghostwalker had them thrown out of the city, and had the CAS take it over. So the CAS section is twice as large as any of the others, and nobody seems particularly happy about it.
ROOTless would have better informationf for you, but if you borrow Janus' Year of the Comet book, I think it's in there.
-kv