Why would my murray lawn tractor back fire through the exhaust when I shut it off?

A good possibility is its running too rich. When this happens and the muffler is hot the motor still turns and no plug fire "you turned off" And the unused rich full goes into hot exhaust pipe and the piston is about stopped doesn't push any more air flow out exhaust. So unexploded fuel air mixtures hangs in the hot exhaust pipe and explodes The mower is to rich on air fuel mixture and can cause this. Dirty or worn valve seat could also cause this explode reaction. "Blow By".
Signed 21 Boat

Moved to power equipment

Does the oil smell like gas and/or is the air cleaner oily?

Hi,
Factories say"throttle it down for 20 seconds or more before shutting it off." Gas build up in a hot muffler is the reason. Shut it off and there is no ignition to burn the fuel.
Peace,
Clarke