The reason the bike backfires is that when you shut the bike off - you are turning off the ignition and a spark no longer occurs in the cylinder.

The engine however is still turning and functioning as a pump - and until it stops turning it is pulling air through the carb and picking up fuel, then the unburned fuel/air mixture gets pumped through the cylinder and into hot exhaust system.

The unburned fuel/air mix in the hot exhaust gets heated up - and if it reaches the flash point it will explode in the exhaust system.

It is harmless - but noisy.  A big single is more likely to do this than a bike with multiple cylinders - as the single piston pumps more fuel air into the exhaust than a twin with smaller pistons pumping into multiple exhaust systems (fuel injection systems don't do this as they stop pumping fuel when the key is turned off).

An excessively high idle speed can make the backfire worse as the engine spins longer before it stops turning.  Lean fuel/air mixture at idle can also make the explosion a bit more violent.

You may be able to reduce the problem by letting the bike idle for half a minute before shutting the key off.