They keep talking about a cascade failure. I am going to give an example to try and explain what that is. Lets say we have a transmission line that is supplying power to a large metropolitan area. The transmission line fails for one reason or another. Since electric power was moving through it you now have two unbalanced situations. On one side you have to much power. Excessive voltage can cause equipment to trip. Part of the grid might try to speed up and pull out of step with the rest of the grid (All generators that are connected to the same grid spin at exactly the same speed. If a system imbalance causes part of the grid to have excess power it can pull out of step. This is very bad news for a variety of reasons I don't have time to go into hear.) All of that excess power has to go somewhere. The system might try to save itself by shedding generation. Typically that is not done with relays but requires operator intervention. On the other side of the system you now have to litle power. All of the generators will start to slow down. Running a generator at lower thatn rated frequency can damage it. Indeed it can damage anything connected to the grid that uses a motor or winding. The system would probably try to load shed. This means it would start shutting off groups of consumers in an effort to find equallibrium between demand and supply. The problem is that each time you shed load you shock the system, sending a huge jolt of power bouncing around all over the place. If other equipment exceeds its operating paramaters it can be tripped off line by it's protective relays, triggering more load shedding. To be honest I do not have any practical experience with load shedding schemes, mine is all theoretical, I have a much better background in generators.
The way the system is supposed to work is that you have excess capacity on line. It takes a little time for the excess generation to ramp up to full power so your system frequency will slow down some but hopefully recover before it's too late.
As an example, one of my customers used to be a cogeneration plant in west Texas. When comanche peak tripped one of their reactors the whole state power grid would slow down. This customer's units were programmed to go automatically to peak load when an underfrequency event occurred. I belive the units would start responding at 59.7 Hz. ( Many generator units have two Ratings Full Load and Peak Load. Peak load is only to be used in emergencies as it is very hard on the machines) Sorry for the long winded explanations but I have had a full evening of watching commentators on tv not stating much useful information. Had to get this off of my chest.