The NCR took over Nevada, that's my best guess, considering I've never played Fallout.
Why would they take over Nevada?
1. NCR was the good guys.
2. That requires Nevada to have undergone some rejoining akin to what NCR is doing.
3. Why would NCR invade Nevada when they could easily just be adding different settlements in California to them, it's not like they had controlled all of California. (Evidenced by places like Vault City)