You turned to pot to stopped. The electronics says speed command = 0. You rotate the shaft, now speed is not zero. Back emf from the motor rotation is sensed by the electronics. You rotated it backwards. Electronics makes a poor attempt to return to zero speed from a negative speed. The supplied instructions (if any) if followed are probably OK, but in a practical world, they are not followed and are useless. You need a safety relay in the circuit, which is no doubt not in the design. I have replaced electronics cards destroyed by the on-off-reverse switch breaking current to the motor. Disconnecting the motor connection directly, while running, will eventually lead to failure of the electronics, when a high current is opened by a switch. Return to zero speed, then switch to stopped once rotation has ceased. Ideally, when stopped, the power from the whole circuit needs to be removed, but now the issue of turning on can lead to high current surges to the filter capacitor destroying something in the input rectifier/switch circuit when power is reapplied repeatedly. The circuit design is no doubt cr4p. A circuit needs to sense the stopped condition, then disconnect the motor or disable the drive circuitry. For startup, the motor needs to be connected before any current is applied. -- The drive circuitry is enabled after the motor is reconnected by a relay. It can be a secondary contact on the relay. For any safety circuit to rely on crappy electronics to ensure no power is applied is a recipe for disaster. USE A RELAY.