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 takes more than a knock to do this - it takes a hell of a force to achieve an AP disconnect this way. In 6000hrs on the Bus, I've never seen it happen. Accidental disconnects are relatively common and is much more likely with an inadvertent touch of the stick disconnect button, or even something falling off the table. As BEagle mentioned, the aircraft is artificially stable and if an inadvertent disconnect occurs, the attitude will stay the same, unless there's applied force to the side-sticks. Recovery is a simple matter of pushing an AP engage button and is normally hardly worthy of comment, except for saying "sorry".
In this case, the brute force both disconnected the AP and applied the pitch down input. I suspect the pilot was motoring his seat forward and the camera applied force to the base of the side-stick. The report mentions witness marks on the camera which indicates considerable force.