The problem is that nowadays in the US there is very little margin in capacity - power plants and transmission lines are operating at close to rated limits. In many areas the margins are in the single digits. Under those conditions problems at single generation units or switchyards can eliminate the margins, and bad things start to happen.
Electric power utilities have load dispatchers who attempt to manage the distribution of power throughout the system. When they see margins falling they start taking various actions to accomodate. They can increase the load on "peaker" plants which burn expensive fuels but can vary their loads rapidly. They can lower the frequencies - running the system's generators just a bit more slowly and slowly damaging your appliances. They can lower the voltages - you'll know, because your CRT displays will suddenly become smaller. They can start cutting off customers in a controlled fashion. Or sometimes they lose control, and you have something like this.
Many things can cause a plant to shut down, or "trip". Here's one example. On days with especially low margin forecasts, the plant was all but closed to all but the most critical operator activity, and we'd even start shutting off lights.
Look out New York:
Current projections indicate that New York City and Long Island will not meet reliability criteria requirements beyond 2001 unless conditions improve. Both areas are transmission and voltage constrained. There are some 10,000 MW of new generation proposed for New York City and Long Island, and the future adequacy of these areas will depend upon the timely completion of these proposed projects.Then again, I won't shed any tears if NY freezes in the dark - they had a perfectly good nuclear power plant at Shoreham. Let them burn antinukers and other watermelons instead.
There's a lot of jargon in this field - here is a glossary.