What happened
Following the start of the number two engine, the captain signaled a ramp agent to disconnect the ground power unit (GPU). The agent signaled that the disconnection was complete, which the captain confirmed by observing that the 'EXT PWR' annunciator light had extinguished and noting zero voltage on the external power selector voltmeter.
After the number one engine was started, the captain signaled the ramp agent that the aircraft was ready to taxi. The agent provided an all-clear signal and directed the pilot to execute a hard right turn. While the airplane was pivoting to the right, the aft portion of the aircraft made contact with the GPU. This impact resulted in the rupture of the pressure vessel within the unit.
The captain noted that the disconnected GPU was not visible from the cockpit during the maneuver.