The clear-air-turbulence sounded far-fetched, now the passengers' lawyers are saying there was a thunderstorm. Presumably, that can be proven one way or another by looking at the radar.

If there was, in an area known for thunderstorms, but the seat-belt sign wasn't turned on, that would be negligence.

It would comfortably explain Singapore Airlines lack of transparency, even if it's not as dramatic as a cyber hack.