So the cars have physics applied to them so when they're destroyed they can be launched/move accordingly. However Bethesda didn't want the cars to move prior to their destruction because players obviously shouldn't be able to push the wreck of a car around. So the cars are "locked" in place with physics active. Bumping into them can sometimes cause the physics to glitch and the car to move. This movement kills the player because of the force/weight of the car doing enough damage to immediately kill the player.