It was caused by using the high level inputs on the sub. When someone slammed a door, the air pressure on the M80 cones caused the cones to move, which generated a signal out over the M80s speaker wires to the 600. This caused the 600 to moan and groan, causing more sound pressure that the 80s picked up hence creating positive feedback. The result was 80s and 600 groaning and moaning together in an orgiastic cacophony of electroacoustic exchange.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated