I think the problem comes about with current control. The driver will see a sudden change in current and try to react. At .5 Ohms motor coil resistance driven by 70 Volts and close to 8 Amps, current control is really important. Plus, FETs are easy to blow out. Most all of the drivers I have looked at do have freewheel diodes across the FETs to protect them from the reverse voltage periods of the switching cycle but it's really up to the control logic to keep things in check.

Dave, don't throw your blown out drive away. I'd like to buy it if it is available.