My condolences on the loss and glad there were no injuries or other property damage.
Got any ideas about what might have caused this?
Was there a known external gasoline leak?
Darn good show and tell about not allowing clippings build up too.
Would be good if the cause could be pin pointed, but looking at that mess I'd say speculation will have carte blanche.

The grass clipping comment is something that needs to be repeated many times.

On my JD LT-155 the housing around the muffler is designed to capture grass. While the manual tells me not to put by fingers into the turning mower blades there is not one word about the possibility that the grass will catch on fire because of the design of the muffler. Because the grass is not obvious, that is the principal reason I got my leaf blower. I use it to clean all of the obscure areas under the tractor.

On the third time I used my new tractor, I was happily mowing and smelled burning grass. For several minutes I did not realize what was happening, but when I did I stopped and was able to get the fire out before any damage was done.

From the picture and because of the apparent melting it looks like the fire started on or near the engine.

Glad no more damage was done.

here is a likely cause of the fire.

There is a fuel hose between the fuel pump and the carburetor. After a few years of service, the fuel line hardens and cracks. Fuel starts to leak from the line. The muffler is located directly below the fuel line. When Ethanol fuel is used, moisture is drawn from the rubber line and contributes to this condition.

If the carburetor was leaking internally, most likely the fuel would find a path into the crankcase.

The above comments make me more aware of gas lines above mufflers.

The fuel tank had the hose on top of the tank instead of gravity feed and the fire never burned the tank. I took the mower for the hydro trans and the rest will bring about $25 at the scrap yard.

See if house insurance will cover it.