Either they had to descend or chose to descend
If the had to descend for what ever reason then its inconceivable that they would not contact ATC
If the chose to descend for operational reasons then again its inconceivable that they would have done so without ATC clearance, hence no sensible explanation
So the non sensible explantation is top of climb one pilot leaves the flight deck for a pee, the other pilot either a) has a problem and cocks it up or b) the remaining pilots decides to end it all for all on board.........by denying access to flight deck.
Locked cockpit doors if so will have killed more than terrorists........