ditto on Larry's question. Sounds like you got a little post engine spontaneous ignition. What you describe is not a "back fire." And is actually encouraged by the AI system.
If you blip the throttle and then shut down you will send a lot of unburned mixture into the exhaust which can combust. If not, and it continues to do it, exercize your warranty and have the dealer look at it. Otherwise, forget it- and just shut down at idle from now on.
Monte
