When flying the MAP manually with a relativly light B737, one easily might apply to much ND Trim in attempting to capture a low level-off altitude. Since the speed might increase rapidly one can find himself in a condition with Stab Trim at 0 / Full ND and T/L retarding to more or less idle.
Yep, possibly confusion over whether the published missed approach altitude of 600 meters QFE still applies after an apparent verbal clearance to FL 80 on the missed approach. The 600 meters constraint is probably still in the in the FMC from earlier discussions on this thread with maybe 8000 in the altitude window. They zoom upward on the miss toward FL 80 but the flight directors and possibly autothrottles still see the lower altitude and give cues to descend. At 900 meters the PF thinks he is 1000 feet above the proper altitude and does the -1 g pushover.
Here's a similar scenario with a Korean Mad Dog freighter out of SHA in 1999:
After takeoff the first officer contacted Shanghai Departure and received clearance to climb to 1500 metres (4900 feet): "Korean Air six three one six now turn left direct to November Hotel Whiskey climb and maintain one thousand five hundred meters."
When the aircraft climbed to 4500 feet in the corridor, the captain, after receiving two wrong affirmative answers from the first officer that the required altitude should be 1500 feet, thought that the aircraft was 3000 feet too high. The captain then pushed the control column abruptly and roughly forward causing the MD-11 to enter a rapid descent.
Both crew members tried to recover from the dive, but were unable. The airplane crashed into an industrial development zone 10 kilometers (6 miles) southwest of Hongqiao airport. The plane plunged to the ground, plowing into housing for migrant workers and exploded.
ASN Aircraft accident McDonnell Douglas MD-11F HL7373 Shanghai-Hongqiao Airport (SHA)