Yeah, the various EEAs and emergency notices are basically opportunities for other participants on the grid to spin up generation capacity and help out.
CAISO encompasses a wider region than just California -- this reflects the state of everything managed by the Western Energy Imbalance Market.
Technically speaking, the Western Interconnection is just a series of interconnected grids on the western-half of the continent. There are some operators that have capacity that can't be used or can't reasonably supply things. If you view the the price map (http://www.caiso.com/TodaysOutlook/Pages/prices.html) and switch to real-time, you can easily spot where the grid doesn't have enough interconnect ties -- the prices will be wildly out-of-proportion with the rest of the system.
The reasons for this are varied: in order to be able to adequately supply load you need physical lines connecting the grid together. One single line isn't enough to actually carry all of the electricity from A to B, because they're physical wires that have load limits (e.g., due to thermals). So while the western half of the US is widely connected, there are spots where more lines could be built to fix the imbalance. CAISO lately has been trying to onboard as many providers as are willing to, because it's a win-win (they get paid ridiculously high prices during events like this and the grid gets more stable).
CAISO encompasses a wider region than just California -- this reflects the state of everything managed by the Western Energy Imbalance Market.