So the plug fouling and everything else that did or did not happen before the second start did NOT contribute to the fire.
With respect to Backpacker, I don't really agree with that. I think that there are lots of aircraft that are difficult to start, or re-start, or some other finnicky behaviour because of lazy maintenance.
These behaviours lead to equally innovative starting proceedures, especially (in the Warrior) throttle pumping, the results of which I've described on here before. Essentially, if you pump the throttle more than about 3 times with the engine stopped, you will have neat fuel runniing down the inside of the cowling. No amount of 'continue cranking' etc will put that out if it ignites, at least until the engine starts.
So there's the point - why does it ignite? Unfortunately, the badly maintained engine that won't start in the first place is the same one that's likely to backfire or have fuel burning in the exhaust. So if your engine is reluctant to start, get it fixed before someone - quite possibly an instructor - devises a 'sure fire' personal technique with catastrophic results.