A bit of idle speculation.
There are, to me, 3 possible scenarios. 1) Sudden and catastrophic failure. 2) Act of terrorism. 3) Incorrect response to upset by flight crew. 4) Suicide.
1) Is unlikely to be caused by failure of the aircraft, albeit memories of the Lauda Air 767 inflight T/R deployment remain. Outside influences could be the one in a billion metor strike, or an accidental shoot-down by armed forces.
I don't buy the idea of onboard fire; in all such previous cases the crew have had time to radio their plight.
After what happened in a Beijing Railway Station last week, I'm sure there is a lot of interest in option 2.