[MadnessASAP]

An autopilot disconnect (caused by a failure of the flight control computer) in an out of trim condition can cause it. I'm not sure how a 787s trim system is built but on our, much older, plane with a mechanical trim system there's a motor that will slowly move the trim tabs to bring the autopilot inputs to 0.

That is if the autopilot is producing a constant nose up control signal the auto-trim will move the elevator tabs towards nose up until the AP pitch signal is null.

It's to prevent fun excursions like this should the autopilot become disconnected without the pilots hands on the controls.