Are you using normally open switch contacts? In that case, you have
open-circuit wires acting as antennas. It would have been better
for a few reasons to use normally closed contacts, so a dirty switch
contact or broken wire would cause a fail-safe condition. It would
also get rid of any floating-wire effects. Probably some capacitors
to ground would solve the antenna problem.

It does not reoccur since I replaced the bad drive and routed motor
leads away from some wiring.