The back-emf from stepper motors is significant - they're good little generators. The protection diodes in the stepper drivers will rectify the back-emf and if you turn it fast enough power up the driver which is clearly bad news. I found that below a certain speed they manually turn quite easily, but when that threshold is exceeded it gets a lot harder. Clearly it will vary significantly with the size of motor.

Easily solved - just unplug the motors.