10,000 feet..aaah...time to relax a bit, accelerate beyond 250 kts, call for a cuppa, and one of you to take a natural break. Normal. (It having been at least an hour since you both strapped in, had your coffee/water, briefed, programmed, closed the cockpit door, run checks, pushed back, taxied, prepared for flight, lined up, took off and climbed with the SID).One leaves the flight deck, the other does the deed.
Just one of a bunch of ideas, but not to be dismissed so readily.
My own guess is a mis-handled rudder hard-over, or a reverser deployment, either of which was probably not recognised and acted upon suitably or swiftly enough. Hazards of aviation.