I have experienced the cross-coupling of the balance canals on a NVFR flight near the coast. A nav over dark land, to a turning point over a lighthouse. The sea mist was just rolling in below us.
We got to the turning point, we rolled the aircraft to start the turn (1 canal) and pitched the aircraft through the turn (2 canals) and then I turned my head to look at the lighthouse below - its beams of light, like fingers rotating in the mist, totally stuffed me up - eyes telling me one thing and 2 confused balance canals, I felt like I was rotating backwards out of my seat, a very powerful feeling. Lucky I wasn't on the controls at the time.
Maybe something similar happened here, he starts a turn, introduces a climb, turns his head to look for a visual feature, feels he is falling backwards and pokes the nose down...