In a control system when you have an actuator that is much more responsive than the load, you can get into instable operating regimes where. On a small scale, when you are setting up servo motors, there is an inertia matching ratio for stability. In some of the systems I set up it was 10:1 load inertia:motor inertia. If you exceed that, you end up with elasticity between the motor and load that the motor can locally be above set point, and the control tells the motor to slow down, while the load has not yet reached set point. The motor is then too slow, and the control tells it to go faster, and you get the motor oscillating around the speed of the load.
On the grid you can have similar elasticity. An inverter many km away can get too far ahead in the AC cycle, then when that signal reaches the local inverter, the local inverter slows down, but the remote inverter has already slowed down, and by the time that slower wave gets to the local inverter, it is already too slow, and so the local inverter starts to speed up. If you get the frequency "right" you can end up with a positive feedback loop. With a high inertia rotating generator, the system is damped and slow enough to respond you don't change set point faster than the signal can propagate.