What could possibly destroy a current limited stepper motor driver?
Motors behave like inductive loads. Suddenly reducing or interrupting current supply on a motor will generate a voltage spike (called flyback) on it's driving lines. The driver can survive spikes, but not forever. The flyback should be eliminated with a diode. You can read more about it on the Wikipedia.

Note: since you can drive motors in two ways, I suggest adding transient-voltage-suppressor (TVS) diodes (one between A and A\, one between B and B) which can elmininate flyback in both ways.