Overvoltage can weld relay contacts together when the relay is opened, even with AC the spark would be considerable. I'm sure that the wreckage was studied extensively to figure out what exactly could have been done to prevent this from happening.
It's one thing to have to design for fickle winds, it's quite another to also have to design for a fickle grid!
Windmills tend to have a destabilizing effect on a grid, if the grid is unable to sink all the power pushed into it the voltage will rise.
Even so, you'd wonder just how much of a rise would be required to cause a catastrophe like this.
Those people filming this are braver than I would have been in the same situation, a mill considerable smaller than this one failed on the island where I lived and the blade was found the next spring. It was embedded several feet down in the soil and about a mile away from the base of the mill!
Dying windmills are dangerous, and to stand there to film it took a lot of courage.
It's likely a design defect coupled with failing breaks and high winds. Modern windmills rotate their blades to keep operating at a range of wind speeds without that winds can apply insane amounts of force which may have lead to over RPMing the generator causing a fire.
Every other windmill is stationary, so they are applying breaks. As to wind speed we don't see a when it actually failed, only the current speed which is not exactly slow.
