My clues:
- A group of onboard terrorists. Tried to take control in unknown target but failed. See United Airlines Flight 93 (9/11)
- Pilot error. See AirFrance Flight 447
- Sicko pilot. Deliberate crash. See EgyptAir Flight 990. Need to check pilot's background. Probably Muslim beliefs
- Plane massive failures .......rapid decompression, wing separate, power, pitot tube, solar flare, etc
- Passenger bomb. Not terrorists but collect life insurance policy. See United Airlines Flight 629
- Unintentional shootdown in SAM military forces except N.Korea....on purpose. Initially theory, see TWA Flight 800. Also, see Korean Air Lines Flight 007. Darn comrades!
- Collision:
** space junk reentered atmosphere
** asteroid
** high altitude weather balloon
** another aircraft i.e. Learjet
- Pilot's fallen asleep. Way off course and eventually ran out of fuel. Or else drunk as a skunk.
- Wrong airport . Plane safely landing in remote airfield like abandon Vietnam airfields, Nepal, N.Korea or beach airstrip located at Amelia Earhart island. Remember Branson MO and Wichita KS?
- Pilot's distraction courtesy of Smartphone/Twitter yik-yak users. See Northwest Flight 188. Or else Twitter was temporarily down thus massive suicide including the pilots.
- Crews orgy in the cockpit. Autopilot accidentally turned off and yoke pushed forward. Yoke was a perfect seat in stewardess position.
- Aliens was the culprit
NW Flight 188
UA Flight 629
TWA Flight 800
Flight 007