If you read the report the Captain was using his camera whilst the co-pilot was out of the cockpit..as he was bored.
At some point the camera was put on the side,the Captain moved his seat forward,and there was a slight input to the sidestick,but not enough to disconnect the autopilot.A short while later the seat was again motored forward,moving the camera against the sidestick,pushing it forward,triggering an autopilot disconnect .
and the following `plummet`.
When `Taking-over control` from another pilot,even if the `other pilot is not leaving his seat,it is incumbent upon you to ensure you are fully harnessed,seat/armrests/pedals adjusted/headset etc/emergency oxygen mask ready/charts/maps all to hand,before you take control of the aircraft;It is your responsibility...even more so if the other pilot leaves his seat/cockpit...And there you remain,until relieved....not pi%%&*g about taking photos/chewing sandwiches ,moving the seat back/forwards,etc,etc.....