Any change in the state of an inductive load causes kickback. If you sever the lines running to a building, the generator side gets the most kick, but even a fu

Any change in the state of an inductive load causes kickback. If you sever the lines running to a building, the generator side gets the most kick, but even a fully passive device gets a kick from the collapsing field in the wires servicing it. The motors generate appreciable back EMF when suddenly unpowered. Likewise an SMPS is an inductive load. Spikes from power supplies going off is probably what killed the disks, assuming they didn't just decide not to power on. Some disks will run for 10 years but power them off once and they don't come back up.