Exhaust gas has very little oxygen in it compared to fresh air: ~1% vs 20%.
It takes oxygen to burn metal.
If a leaky or too-short exhaust pipe lets fresh air reach the exhaust valve it burns.
The exhaust does not come out of the engine in a steady stream, it comes out in pulses.
This is pretty obvious with a single cylinder engine, like on a lawnmower.
When the exhaust valve starts to open the pressure in the cylinder is still pretty high.
So the exhaust gasses leave rapidly early in the exhaust stroke.
This creates a wave that travels down the exhaust path at the speed of sound: 1100 ft/sec.
Also, when a pressure wave traveling in a pipe reaches the open end (or a chamber, like a cat converter or resonator) a rarefaction (negative pressure) wave is created and travels back in the other direction.
At low engine speeds these waves can travel up and down the exhaust system several times on each stroke.
So exhaust gas can flow ‘the wrong way’ at times.