[Bduggan04]

I would expect behavior like that from a short in the wiring. You could be back-feeding current into the speed controller for the unconnected motor. I have never seen a current like that induced in a wire on a FIRST robot, so before you chase the rabbit with the emf meter, check your wiring. Using the ohm checking capability of a multimeter, check for continuity where there shouldn't be continuity (i.e. motor wires on opposite sides of the drivetrain)