Most likely extra fuel left in the exhaust pipe. When you go to restart it it burns the gas in the exhaust quickly and thats where you get your backfire. When you shut it off let it idle for a moment before you shut it off.
Mine just backfired for the first time this weekend too! It was loud and I wans't ready for it. It did it on the pull that started the engine. My belly pan is pretty clean, but only have 151 miles on her anyway.
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