I suspect the crew disconnected the auto pilot in order to initiate a rapid climb. My guess is that they were attempting to avoid weather at the last minute. They had requested an altitude change to avoid weather but ATC had denied the request due to congestion at the higher altitude. They probably hit some nasty stuff, said to hell with it and reefed back on the controls. At that altitude there’s only a few knots difference between going transonic and stalling. A 6,000 fps nose up climb would most certainly have stalled the aircraft.
This is looking like pitot freeze up.
The air trapped in the system expands with altitude, effectively fooling the autopilot that an over-speed has occurred, and commands a climb to reduce speed. That only causes the trapped air to falsely shows a additional speed increase, thus commanding more pitch up.
Finally the Autopilot gives up, and disengage.
Leaving the pilot hauling back on the yolk to reduce the false over-speed.
The angle of attack is so large the they get an engine compressor stall and the airplane,yaws into the dead engine, with a swept back wing then stalls and the plane enters a flat spin, possibly on its back.
It happened before , have a look at “Birgenair Flight 301”
( and that was a Boeing )