Exactly what Cisco said which was the first thing I said (pretty sure the first reply you got as well)
Hot exhaust gasses + hot premix blown out the pipe + paper = fire. It's absolutely not a question of if but a definite matter of when.
I sent a car to the junkyard because of a spontaneous engine fire. Granted it was a POS and that was a known issue on the 2005 Impala... the valve cover gasket leaked oil into the spark plug well, moving down the road kept it just cool enough, parked for less than 5 minutes and the engine bay was hot enough to spontaneously ignite that puddle of oil. Considering the intake, valve covers and a bunch of other crap on top of the engine was just plastic
GAME OVER