Death randomly occurs when exiting certain planes, such as the Fokker, giving the message (in chat) "death.plane". I cannot identify the specific set of circumstances when this will happen, but it's frequent. It has also occurred with the Spitfire, but I haven't yet tested all the planes.

Side note, I also tested using a custom plane with the Biplane model and collision boxes. In this plane, exiting it when it had no fuel resulted in consistent death. When I partially fueled it (but did not accelerate or take off), exiting never killed me.

And more thorough testing suggests that this death occurs when a plane attempts to place an exiting player into any non-air or water block. In this case the offending block tended to be tall grass.

I'll close this and post a more intelligible version.