Two thing that have always struck me odd about this are the number of bodies that were found, and the absence of life-jackets.
Seat belts don't fail - they will carry you to the bottom. Those bodies that were found were therefore presumably not restrained and were able to float free of the disrupted cabin as it sank. There were more unrestrained people than you would expect in moderate turbulence with "all passengers and crew returned to their seat with seat belts fastened". There were almost certainly more unrestrained people than bodies found - some bodies were simply never found or sank over the following days.
To me this suggests that the interval between normal mild turbulence (with people still walking around the cabin or seated without belts) and sudden loss of control (which incapacitated people such that they could not get to any empty seat) was very short. And that control was never regained.
Also the life-jackets. Hard to believe that no one had a jacket on if there was any semblance of control on the way down. The crew would have told people to put jackets on, and some would have - some likely would have even in the absence of any instruction. They would have all known that it was the middle of the night in the middle of an ocean. Again, maybe the loss of control was so serious and so sudden that no one had an opportunity to put on a jacket over the following minutes. Hard to imagine, but it is difficult to support any other conclusion. I have seen speculation that it was an attempted ditching, or they were recovering control and lost visual reference and ran out of space. If either of these were true, I think we would have seen some jackets.