I know one thing that would make a light 737 go up like a rocket, and make it hard to control; A WS escape manouver with throttles to the walls.
That is how we train it in the simulator. We also normally set the MCP altitude up (just spin the wheel) to avoid altitude capture and confusing FD commands.
IF you get altitude capture and try to follow the FD commands, now commanding nose down with engines producing (27K ?) max thrust, things could go pear shaped really fast.
6 hours into this flight, at night, bad weather, tired, a number of holes are lined up.
For those European pilots who claim their rosters are just as bad. Do you routinely fly night turn arounds? Four hour flights into the night, then back again? 8 days off in a month? Switch between day and night duty?