One problem that happened here is the _voltage_ spikes as the synchronous generation went away. Voltage _spikes_ on generation going away seem insane, but it's a real phenomenon.

The problem is that the line itself is a giant capacitor. It's charged to the maximum voltage on each cycle. Normally the grid loads immediately pulls that voltage down, and rotating loads are especially useful because they "resist" the rising (or falling) voltage.

So when the rotating loads went away, nothing was preventing the voltage from rising. And it looks like the sections of the grid started working as good old boost converters on a very large scale.