After high rpm operation, the pipes are very hot. This 'hot' condition of the pipe increases/improves pipe scavenging by making the cylinder run slightly leaner than richer... Which in turn will cause slightly higher idle rpms until the pipes cool down.
Blipping it with the choke makes the fuel/air ratio rich, and a rich mix will run cooler, which in turn will also pump out cooler exhaust gases... With choking, the pipes cool much faster than waiting, and that's why choke-blipping works.
If it's idling high rpm when you shut it down, the motor spins more revolutions than compared to a slow rpm idle. This causes raw unburned fuel to become vapourized and end up in the pipes.
Then... Next time you start it up, the raw fuel vapours in the pipes can ignite, and you get a healthy bang out of the exuast system!
It is perfectly normal for a carbed engine, because carbs cannot adjust according to sensor inputs as EFI would. Just blip the choke or wait a bit longer for the pipes to cool, the rpms will drop, and minimize the chance of 'the BIG BANG' when you start it up again.
:chug: