When no tether is connected, there is very little resistance between the ROV and control panel. When thrusting and then letting up on the joystick, the motors act as generators and feed power back into the 48v power supply. This causes the power supply to shut down to avoid what it detects as an overvoltage situtation. The system then loses communications between the ROV and control panel and will need to be rebooted to reestablish communications. This doesn't cause any damage to the system.
When a tether is connected or when the propellers are in water and have some resistance to spinning freely, the feed back voltage is below the overvoltage threshold and the problem does not occur.