I would think that when the circuit is broken a couple of things might happen:
1. There would be a spark in the switch causing a transient into the motor coil.
2. That transient may be high enough to to cause a short in a moist or already compromised environment.

Maybe one of your motor coils has failed.

I had a similar thing with one of my CNC machines. It would run happily for hours on end and only trip when I turn the main power off. After much stuffing about and replacing many components did it come to light that the coolant pump had failed and I suppose the excessive magnetic field built up in the motor coil with the addition of the switch opening transient field was enough to short the pump to ground.

Dunno - may be I am talking rubbish but it seems to be the only thing that makes sense.