Speculative yes. Highly speculative maybe not as much. You need to explain a 20,000 feet vertical drop in less than 2 min. Might not be the fault of the airline. Could be Boeing unclear instructions or a Supplier providing a faulty piece.
I bet that kind of drop will inevitably exceed the Vne because this plane does not have adequate speed brakes for this kind of maneuver (and why would it). I'm surprised it was still even intact when it hit the ground but according to the dash cam videos it was.
But I also think there's many things that can cause this. A terrorist taking over, the FBW system seriously messing up.
Anyway the investigation will tell us. Maybe soon because the black boxes won't be far away.
Not such a good point, given that one indeed survived, and that in that particular case it wasn't the impact that destroyed the other one but the post crash fire.
Black Boxes are designed to stand 3,400 Gs what is about 300 Mph. Maybe the airplane did hit the ground at close to those speeds.
And the fact that the second one did not survive the crash due to fire, shows they are not indestructible. Fire is actually what they are supposed to survive for at least 1 hour.