Spontaneous combustion

I've heard of it before but never experienced it. Till now. Spontaneous combustion. Ain't that the shits! Poor Greg was working on a rib for the boat when he smelled something. Turning around from his work bench he noticed a fire thru the windshield of the boat. After a second or two he realised the flames were on his side of the glass! He had the good sense to put out the flames by smothering them with a blanket. ( He's my hero ) The fire started in the vinyl bag the fish finder was in which was sitting on the dash by its self with nothing else around it. Now there is no battery in the boat, the fish finder doesn't take batteries, there was no ignition source for the fire, and neither one of us drinks or does drugs ( any more ). What could it be other than spontaneous combustion? I pulled the fish finder apart and repaired it so it works now ( although it ain't pretty anymore! ) but I had to call my buddy Robert Moriarty to make new windows for it. The heat raised a bit of hell with em! If any of you have a good explanation or theory on how the hell this happened, please, share it with us!