It almost has to be one of the pilots very forcefully pushing the nose down. The 737-800 autopilot won’t do such a drastic maneuver.
Hard Nose down is an anti-stall maneuver if wings departing flight. Might have been overreaction to stall horn?
The autopilot was off; I’m not a specialist, but it looks like the crew’s error. The Russian authorities has ordered all the Russian pilots flying Boeings to take up a training course simulating the scenario.