Maybe simply in his haste to grab the side stick when the autopilot disengaged would easily do that since he was reaching for the sidestick in the dark from the left side of the stick and probably bumped it to the right. Unlikely turbulence would have caused it. So far all we know is they lost airspeed indications so the autopilot and autothrottle disengaged. I am sure his hand was nowhere near the sidestick when it disengaged and with his limited experience in the aircraft doubt if this ever happened to him before this event. Sometimes it is nice to have a big fat yoke in front of you when things go south instead of a meal tray.