I already acknowledged this was a speculative line of thought. Don't think any air crash investigators are looking into a nobody on the internet for accident clues, so it's a kind of very limited irresponsibility.

However there are actually not that many causes, that could explain the vertical drop either than, and in order of decreasing probability:

- Suicide/Terrorism

- A Jackscrew Mechanism Failure: https://www.wikiwand.com/en/Jackscrew

- Accidental Deployment of the Reverse Thrusters.

The accidental deployment of the reverse thrusters, is something that requires multiple mechanical failures, is not supposed to happen but Boeing already has a past history with it, although on a different model. The Niki Lauda/Boeing controversy.

"What if the reversers OPEN in flight?!"

Video at correct time: https://youtu.be/OguKthkuaT4?t=1026