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.