The valve warping explanation makes more sense than the first lines of my last post re. oxygen.
Otherwise the Overrun Fuel Cutoff would be a problem.
When the engine is above idle speed and the throttle is closed no fuel is injected or burned, so more oxygen goes through into the exhaust.
At any rate fresh air reaching the exhaust valve is bad news.
"But once any length of pipe is in place...I don't see it"
The open end of the pipe (or a leak near the valve) can suck in fresh air when a rarefaction wave reaches it.
The gas in the exhaust pipe can move back and forth in both directions even though there's a net flow outward.
Kind of like a organ pipe.
So the air could make its way back a few inches to the exhaust valve.
A leak between the exhaust manifold and head is sure close enough, and maybe the outlet of the manifold if it's compact.
Probably happens more at certain engine speeds.
"I drove my Bronco for well over 150k miles with straight pipes, no cats, no mufflers, didn't burn a valve or anything"
Pipes ran to the back of the truck? That's well over 10 feet.
No way air makes it that far up the pipe.
I guess airplane engines can get away with short stubs because they run at a steady high output.