AFLM, it was the result of one of the substations out in Yuma, AZ shutting down, which transferred to load to the next power station. It promptly overloaded and shut down, which transferred the load father down the line shutting down distribution and power generation stations like a string of firecrackers. It's the same thing that caused the 2003 blackout of the NE United States.

The total area affected were:
Yuma, AZ (partial shutdown)
San Diego county
Riverside county
Orange county (partial shutdown)
Tijuana, Baja Mexico (???)

I got caught in it and was lucky in that I was able to stay at work and missed most of the traffic.

Lessons learned:
The following utilities went along electricity
*Cash (ATMs)
*No gasoline (Pumps inoperable)
*No credit card transactions unless store has backup generator.
*Severe water restrictions (pump stations operating a reduced capacity. Areas higher than the reservoirs were in trouble)
*Boil water order (Water filtration facilities at reduced capacity or inoperable)
*Cordless phones (exacerbated below problem)
*Cell phone network (Surprisingly the network stayed up, but it was so overloaded that only text messages went through reliably)
*Local radio stations (One emergency station, AM 600 KOGO, stayed up)
*Trolley (operates off of grid)
*Traffic control (No working traffic lights, and everybody left about the same time)

Panic buying of the following items:
*Dry ice (sold out in first hour)
*Batteries
*Bottled water (was 100F earlier in the day).

Things we did right:
*Generous supply of candles. Only had to use flashlights if out of the main room. IMHO 6 candles are enough light to have dinner by and still read.
*Battery powered radio. (+)Helped keep us abreast of developments. (-)The blame game started only 5 hours in cutting into useful information.
*Flashlights, spare batteries, and bottled water already in kit.
*Both cars had plenty of gas (I maintain a 1/2 tank minimum in hot weather due to brushfire evacuation panic buying of gasoline)

Things to add to kit.
*Candle lanterns (area lighting was a problem and 1 candle = approx 1 min-maglite)
*Extra cash