heat tape wasn't on a dimmer switch. It was absorbing max voltage.

max voltage+ reduced airflow in a rack setup+ age of heat tape= malfunction.

To properly setup heat tape (according to Bean Farm), you must regulate the temp to below 95 degrees. Then use the thermostat as a fail safe for overall temp of the tub.

Surges do not have to be huge to cause damage. Small spikes rarely trip surge protectors. This is the reason many computer guys have dumped the common surge protectors and changed to power conditioners.