Originally Posted by Fabrik8

Possible speaker wire shorts, an overheated sub, or a loose ground. Subs that have been overheated (from too much power or distortion) can have broken down insulation on their voice coil wires. Loose grounds can make fairly high current transients when they make and break contact. There are a few possibilities. If the amp is on speakers with a crossover (instead of a sub) the crossover could have a problem.