It surprises me that there isn't a requirement to hold the stick constantly in position for a period of time to disconnect the autopilot, not simply for a knock to do it.
It reminds me of a Cessna 182 fatal crash at Leicester, the pilot on take off opened the throttle and his hand accidentally turned the autopilot on which was situated above the throttle, the elevator trim then ran all one way unnoticed during the roll and on lift off it climbed and stalled killing them both.
The autopilot system was later modified to require a press on the power button for a couple of seconds to operate it, additionally they introduced a verbal trim warning that you couldn't miss.