I can how the 31-deg pitch-up occured, but how did the following 76-deg nose-down pitch movement (to 45-deg aircraft nose down) occur? Surely a stall at 31-deg but with full power on a powerful aircraft would not need such a large nose-down movement, or for the aircraft to pitched down so far to accelerate back out of the stall.
Put yourself in the position of the 2 pilots. How much of a shock do you think it would be to suddenly find yourself in a highly unusual attitude in poor weather, at night, and at low speed?
Even on departure, 18degs nose up is a 'normal' maximum. It is planned and expected, but still feels steeper than it is. I'm sure 31degs felt more like a vertical climb and perhaps led to an extreme recovery manoeuver. Big a/c, powerful engines, lots of inertia - very easy to see how it could happen once a crew becomes scared and disoriented.
Very few operators have an unusual attitudes recovery training programme, yet accident investigators have identified loss of control incidents (presumably resulting from UAs) as the major factor in fatal commercial accidents in recent years.