There's a few factors that could lead to widespread damage. The other commenter addressed grid frequency which is also important, but the real issue is over dra

There's a few factors that could lead to widespread damage. The other commenter addressed grid frequency which is also important, but the real issue is over drawing current. (Disclaimer - I am not an electrician). In most houses you have a fuse box or breaker panel, this is to prevent the appliances from drawing too much current. Conductors heat up according to their resistance and the current applied to them, so if a circuit with 14 gauge wires suddenly has a load of 2000 watts applied to it, it will begin to heat up. If this load is applied for too long it can get hot enough to start a fire inside the walls of a house. This is why we have breakers - they will cut the circuit if it exceeds a safe level for too long. Power grids don't really have breakers. They have complex control systems and highly trained operators to serve the same purpose. If transmission lines have too much current applied to them they can expand and sag, catching on other wires or structures causing damage and outages. Overloads can also damage substations and transformers for similar reasons (heat, thermal expansion). In this case, they did exactly what they had to. They cut the power to millions of people for a few days and saved their infrastructure from months of repairs if things went catastrophically wrong.