That would be so stupid it would deserve a Darwin award! And it's almost too stupid to even be remotely conceivable... yet I've seen/almost been a victim of something not too dissimilar. My friend's own (older!) brother thought it was quite hilarious to jerk with the driver's seat (by twitching the thing that controls the angle of the seat) whilst his own brother was driving at 70 mph (*cough*) on a 🤬 motorway... (and I was in the passenger seat in the front) - I wouldn't have believed it if I hadn't a) seen it with my own eyes and b) experienced it with my own sphincter...
Yup. Lots of passengers will grab the handrail or door handle to brace themselves when cornering, especially if the driver is cornering with enthusiasm. When I was younger, I had an occasional passenger who had this habit, except she wouldn't grab the handrail, she'd grab the handbrake

. She didn't ever actually pull it but all we needed to do was hit a bump or something that made her engage the handbrake by accident and we'd have been hedgeseekers.
Another friend once opened the back door open while we were moving and pretended he was about to jump out. That was distracting. If I'd looked at him, then looked forward again and been surprised by something jumping out of a side street, and then instinctively swerved in in the right direction, he really would have been ejected. As it was there was no such emergency. I told him to close the door behind him on his way out, we all had a good laugh, and drove home.
I've seen a girl passenger who used to like popping the car out of gear, while cruising, just to annoy the driver.
And it doesn't even have to be a prank gone wrong. Innocent things can get you.
A passenger in the back seat, not wearing a seatbelt, or even an unsecured pet, stands a very high chance of killing a front seat occupant in the event of a frontal collision at legal speeds, by either pushing their chair forwards and crushing them against their own seatbelt, or delivering a blow to the head as they fly past en route to the windscreen. It happens a LOT. How many drivers here are happy to drive when the passenger sitting behind them is unsecured?
How many of you, when packing a boot, ensure that stiff objects like poles or struts or whatever are not aligned in such a way that they will puncture the rear seat and skewer a rear seat passenger if you get rear ended? How many of you keep loose stuff in your car that turns into missiles should you roll?
Even those trying to be safe can put you and themselves in danger. Someone saying to something like "watch out for that truck" or some other arb warning will easily distract you from whatever it is that you were paying attention to, and waste time while you try and figure out the specifics of their warning. What truck? I can see three. And all of them are threatening. Oops! That van I was originally watching pulled out in front of me while I was looking at trucks. Bump.
It's easy from here to say that the driver (or someone else in the car) had been terminally stupid, but had we actually been there, it might not have seemed so stupid until something unexpected went wrong. On the other hand, we could of course had been standing there thinking "that's a stupid idea. They're going to kill themselves."
It's usually the thing you don't expect that catches you out. Not the event itself, but an unpredicted side effect.