How is a spike from the motor that was caused by the switch opening getting back to the motor through an open switch?
By arcing back across the contacts as they open? The voltage developed by rapidly dumping current through a motor winding can easily get up into the several kV range.
I've repaired several VFDs that got wiped out by this kind of abuse. In one incident, it was a simple safety disconnect switch at the motor that killed it. In the others (machine tool retrofit applications), it was the installation (by a well-meaning machinist) of the VFD upstream of a reversing contactor or drum switch, to try to keep as much of the original setup in use as possible. Those problems were solved by wiring the VFD right to the motor, and handling the reversing at a low level, with a logic input to the VFD.
