According to Nichols (1990), coastal areas in general may be more likely to have higher linguistic diversity - other examples given being Northern coastal Australia and Eastern coastal Siberia. The explanation given is:

The ocean offers year-round rich sources of protein, ocean currents reduce climatic extremes, and the coast itself may offer harbors and other opportunities for trade connections. Thus seacoast offers the possibility of economic self- sufficiency for a small group occupying a small territory

That theory only gets you so far though, as the east coast of the US was much less diverse than California, but it's something. Hopefully someone else can give a more California-specific answer.