He may have lots of deposits in his muffler/exhaust system that continue to glow a few seconds after shutdown and ignite the fuel/air afterrun. Just a thought.

John, I have operated for 4.5 years, 600 hours and never shut my fuel valve off after the flight was over. I have left it ON for maybe 3-4 weeks during the winter bad weather in between flights. Never had a problem or a float bowl overflow. I did this because years ago someone said its better to keep the whole carb wet with fuel when using Mogas to avoid letting it dry out and varnish deposits may want to form, and seals/gaskets need to stay wet. Note: this is not an excuse to keeps months old Mogas in the tank, because it can go bad. I don't remember who suggested this or how knowledgeable they were. A few weeks ago I mentioned this on the Rotax-Owner.com forum in answer to the same question you asked, and I got criticized by several of the gurus on that forum who said the fuel should always be shut off at the end of the flight. Its too dangerous in case of a needle that doesn't seat and overflows the bowl out the vent tubes. They didn't think my reasoning was valid or worth the risk. So go figure your own opinion. Actually I see their point (even though I have had no trouble for a long time) and have changed my mind and am now shutting off my fuel.