Believe it or not, this is a common occurence. If you do a search on this BB, you'll read the stories.

I had a friend who experienced the same thing. His smoker went up in flames on his deck. It destroyed the deck and some of his house. It was a real mess. His story was almost identical to yours. In fact, he had placed his Big Chief smoker on bricks so it was about 8 inches off his deck. He had been doing that for years. But for whatever reason, the burner got hot enough to ignite his deck. He was very lucky he didn't lose his entire house.

Take comfort in knowing that you're not alone in this experience.