But, that's the whole point, ain't it.

For some reason, the other generators in the grid couldn't take up the slack of the loss of one generator. That should have been the minimum operating condition. The system should never ever have been allowed to enter that region of operation where there was no net.

The obvious reason is that the power companies were running the generators over capacity, or in an unco-ordinated manner. Deregulation enables, nay, virtually requires, this sad state of affairs.