A positive electrical wire connected with the frame creating a short in the electrical wiring causing what you described as spontaneous combustion.Whatever it is - fix it!
Last summer, my JD X740 was parked in my airplane hangar, along with all my other toys. Whatever the fault with the X740 was, it spontaneously ignited at 4AM, and burned down everything. Don't "tolerate" electrical faults!
A couple years ago I was mowing with my old faithful 2084 Cub Cadet. I was mowing beside a building where I had mowed for years and the next thing I knew the mower was turning over. It landed on its side trapping my left ankle. I was laying there struggling to get my leg free and thought all I need is for this thing to catch afire. About that time I heard a poof sound and sure enough it was smoking and burning. I pushed and jerked with force then and freed my leg. Better scratched and lacerated than roasted.
I doused the thing with water to put out the flames. When I turned the thing over the battery slipped out of its holding strap allowing the positive battery to contact a metal part of the frame. This caused a short in the wiring. The hot wiring ignited the plastic side panels and hood, etc. The mower was a total loss.