The way I understood it, Demand spiked as a couple plants went off line. So they immediately started shutting down parts of the grid to lessen demand.
I am guessing that means the operation of the grid has to be watched and manually operated. If demand is too high and nothing is done, the power plants will go into “overdrive” trying to meet the demand and can fail.
It was said if the blackout of parts of the grid was not done, it would have taken months to recover. So I am thinking there would have been some damage to any power plant online at the time to repair.
That is mostly guessing though, there hasn’t been a clear explanation, so that is my best guess.
To get really confused, wade through this one and try to figure out what happened: