I am going with what happened on the Air France 447 flight. Inexperience, questionable weather and instrumentation problems. By the time they figured out what was going on they couldn’t recover the aircraft.
Flight tracking website flightradar24.com says it lost contact 40 minutes after take off from Kuala Lumpur, not two hours as previously reported.
You can take the inexperience out of that. The 55 year old captain on board had more than 18,000 hours flight time.