The fast idle bike in flames thing has been documented on here at least once, though I don't remember if anyone figured it out that time either.
When was the last time the QDs had been apart? I know that maybe one in ten times I put one of them back together, I might nick an o-ring and start a small gas drip. That combined with a hot header/cat could certainly do it. The thing is, when the bike is running, those small drips usually turn into a lot of gas, so I would think it would have been pretty obvious when stopped.
I hate that it happened to you though. Any plans for a rebuild?