Suicidal Pilot Theory
I'm with Ngineer on this one. One or other of the pilots has likely left the flight deck for one reason or another and was locked out by the other pilot. It would be a simple matter to disable the comms, don an oxy mask and depressurise the aircraft so as to render all other occupants unconcious despite the "rubber jungle" deploying as it has only limited duration. One pilot on 100% oxygen is going to have far greater supply than the supplemental pax system will provide.