Wires behave differently when supplying AC power over very long distances from multiple power stations to many different loads. You have a given force from your steam turbine or whatever turning a generator which is frequency matched to the grid. If the grid doesn’t use as much power the turbine is still supply the same force so the generator spins faster and the grid frequency increases.

Impedance losses for example are proportional to frequency, and a generator spinning faster also produces more waste heat etc.

PS: Many systems are designed to avoid this situation, but a prior assumption is this stuff has failed.