It's hot out there -- lots of AC running, no doubt. A sudden shutdown of a major power plant would probably upset the grid something awful, especially if they've got transmission line problems, too.
NERC policy is designed to prevent just this scenario - a scenario where the loss of any single component (be it a plant, power line, or whatever) can take large parts of the grid down. Utilities are supposed to be following those rules, but I've seen companies taking chances the last few years (though my present employer doesn't seem to follow that trend yet).