A Singapore Airlines flight from London was diverted to Bangkok, where more than 70 people were being treated for injuries.
Recent research indicates that turbulence is rising and that climate change is a cause, specifically elevated carbon dioxide emissions that affect air currents.
Heart Attack / Epileptic Seizure induced from massive fear/stress/anxiety.
Heart Attack is more likely, but a violent enough Epileptic Seizure can snap tendons, ligaments or your neck.
Edit: You could also possibly choke to death on your own vomit.
All those could kill you while secured properly.
Edit 2: All sorts of other stress induced cardiovascular stuff like a bursting aneurysm or irregular heartbeats for a long enough time untreated could also lead to death.
I have been in a few car accidents in my life (none my fault).
Got whiplash, bruising... but the main thing is the intense shock, literal medical shock. Like you are not actually there, but at the same time everything is hyper real... its hard to describe.
As the adrenaline wears off you realize your heart is pumping like mad.
Its... its Donny from the Big Lebowski. Sometimes something that many people would just be shaken from, need a week off to recover... unfortunately, lots of people can't handle that stress, and their heart will give out.