Here's what Tom Cumberland had to say:

"MoJo is right. The very high power of the amplifier makes it suseptable to line surges. During mid-west and florida lightning storms the AC power lines outside the house are going up and down trying to regulate the line voltage.
This would normally not be a problem as most subs don't go down as flat as ours does. The amp sees these as audio from the point of view as voltage variations, hence the thumping."