only thing I can imagine is that there is enuf gas fumes/oxygen ratio and have it enter the mod ( mech) and the switch is activated causing a small electrical arc on depressing the button ( make/break connection arc) the fumes in the mod could ignite thus igniting the fumes surrounding the mod and then everything else but to get that density of fumes into the mod and also in an enviroment with dense enuf fumes to promote flame travel to the fuel source.... you would have to be holding the mod next to the fill hole on the vehicle trying to make it go off.... it is probably more dangerous using a cell phone while filling up the tank....
It was my experience in 36 years as a tech rep that high voltage switches (24v to 120v) failed with arc related contact pitting many times more than low voltage (5v) switches that just delivered a pulse to the logic. I'd pull microswitches apart after I changed them to inspect the failure.
In a vv/vw mod the switch is just delivering a low voltage pulse to activate the logic. In a mech, there could be a small disconnect arc since the amperage is higher. Still, it's enclosed and the gas fumes would have to infiltrate the switch housing in heavy saturation mode and have a heavy vapor trail to ignite all the way back to the source. At that point, you'd smell something like sniffing gas right out of the can.