dude checked out way before the flight even pushed back, so it likely was planned. However, i doubt investigators will recover any evidence of it as i assumed he kept it to himself.
as for the timing, it was roughly 25-30 mins after departure, so at a relatively low stressful stage of flight. Take-off up until cruise and descent till landing are the most stressful stages of flight and requires all of the crew's undivided attention. So the captain was totally fine and legally allowed to leave his position for the bathroom and i'm thinking the first-officer even recommended he do so (hence the sound of the chair moving back). It was at this point that the first-officer locked the door and initiated the descent.
See that makes me wonder, if the pilot hadn't left and waited was he just going to crash into what ever he could possibly kill more people? I believed it was planned too. I was wondering was the plan for the mountains or just where ever he could. If the first officer never left would he of just waited until the next flight. Crazy and extremely selfish. Do you fly?