It's a grid. So multiple powerplants are essentially linked together.
Think of it this way. You have a "network" of transmission lines that supply power to towns.
The powerplants put power into this networks and the towns basically take power out of it.
This is done so that if one powerplant needs to go down for service or a fault, the others can step up the power and no-one notices their local powerplant is offline.
Seems that Thursday afternoon, they had some sort of power surge that resulted in a cascading series of failures that caused a lot of powerplants to go offline.
Since we use AC (Alternating Current) in North America, and it has to be at 60Hz, all the powerplants on the grid have to stay in Sync at 60Hz. If too many plants go off line, you cannot keep the generators producing 60Hz current at suddenly higher loads on each generator.
Since the generators can be damaged (which will result in a much longer outage) the generators are taken out of the grid.
While it was a pain yesterday for 50million people, the grid really does make your electricity much more reliable.
TB
because there was nothing else to do