What happened
While the aircraft was en route, the captain moved to the main cabin for a rest period, and the reserve first officer assumed the captain's seat. During this time, a flight attendant entered the cockpit to deliver a box of beverages to the flight crew. Following instructions from the reserve first officer, she attempted to place the beverages on the footrest of the center observer's seat.
As the flight attendant struggled with the placement, the reserve first officer noted that the first officer's seat was obstructing the area. Without notifying the first officer, the reserve first officer reached across the cockpit and activated the horizontal movement switch for the first officer's seat, moving it forward.
At that moment, the first officer had his legs crossed. The movement of the seat caused the first officer's legs to contact the control column, pushing it forward. This input exceeded the limitations of the autopilot system, causing the autopilot to disconnect. The aircraft responded to the forward input by pitching down. The event resulted in injuries to passengers and crew. Following the autopilot disconnect, the first officer took manual control and successfully returned the aircraft to level flight.