The claimed cause is interesting: Induced Atmospheric Vibration.

Basically, if the conditions are just right, at the corona discharge threshold, the force of corona discharge leads to an oscillation with the corona discharge turning on and off. This in turn causes oscillations on overall power discharge (corona discharge can be a non-trivial amount of losses), triggering protection circuits and causing blackouts.

At least, that's the one description I could find.. but it sounds plausible.