Electronic – Stepper motor disconnection from drive

Turning the system while energized will cause high currents to flow, but disconnecting it while operating will cause high voltage as the motor's magnetic field collapses. The high voltage is probably what will damage the driver. It is designed to handle high currents.

But turning a stepper like that is generally a bad idea. From Jones on Steppers:

It is also worth noting that the best way to demagnetize something is to expose it to a high frequency-high amplitude magnetic field. Running the control system to spin the rotor at high speed when the rotor is actually stalled, or spinning the rotor at high speed against a control system trying to hold the rotor in a fixed position will both expose the rotor to a high amplitude high-frequency field.

The voltage coming out of your motor probably is AC, but that is being rectified by the kickback catch diodes in the driver circuit. Assuming this circuit was designed and wired up correctly, only the correct polarity voltage will be fed back onto the DC power input net. As long at this voltage doesn't exceed what the circuit is rated to operate at, there should be no problem with this.