At an altitude of 900 meters the crew pushed the yoke forward and the stabilizer was put 5 degrees into dive, causing the plane to go into a vigorous descent with the vertical acceleration up to to -1 g.
The answer may lie in that statement. They pulled negative, and I have seen many strange reactions to negative, especially as many new generation pilots may never have done aeros. On one occasion turbulence on approach caused a momentary 0g, and the f/o let go of the controls, screamed, and grabbed the ceiling. Bit of a worry. On another occasion in training a short 0g was confused with a stall, resulting is a substantial push forwards to 'unstall' the aircraft, resulting in -1 or more.
In this case the input of 5 units of trim is not normal. That is a lot of trim, even if the flaps are down (works at 3x speed). There is no reason to use trim like that if pitching forwards, as the cc is rarely that heavy that you need to unload simultaneously while pitching. Someone should see how long it takes to run five degrees, with the flaps down (no longer on 737). There are elements here pointing towards a sudden pitch to catch an altitude, that developed into a new unusual experience -- a 0g float -- that provoked a wrong reaction.