It seems to me if a factory defect caused the fire with proof and the equipment was still under warranty with claims done by the dealer why it wouldn't be covered? The key words here are factory defect not the owner lack of maintenance like a bird/ rodent nest on the muffler. I have rodent damage/repairs come into my shop often ever year. Just got done repairing the wiring harness on my own car that mice ate up.
Even if the mower was out of warrantee if a fire was caused by defect then the mower company could, should and ultimately will be liable.
If a fire is caused by a bad factory routing of a fuel line the mower company will be held liable.
This is why the mower companies are forever sending out dealer notices & recall notices to change position of fuel lines.
However every fire I have had anything to do with was 100% the fault of the owner.
No1 filling a hot mower with fuel, usually not from an authorised fuel container, usually without any filling aid ( funnel or spout ) and many while the mower was still running.
No2 is failing to keep the engine bay & deck clean. A lot of us have made a fire by rubbing a stick on another stick in a pile of dry grass or schredded bark, yet we think the belt can run submerged in dry grass without any problems.
No3 is rodents / birds nesting around the engine & muffler.
No 4 other stupidity.
The one that comes to mind was a customer whose neighbour fitted a belt that was way too small to the PTO.
This caused the PTO bearing to collapse, then the wiring to short out , igniting dry grass soaked in deisel that the owner used to degrease the mower.
The owner was 86 with poor eyesight & hearing so he did not notice the rageing fire under the mower, drove into his shed got off walked out and went inside for a cup of tea.
Luckily both neighbours had noticed this , jumped the fence and were busy saving the contents of the shed ( farm with lots of gear in there ) when the fire brigade arrived.
Despite this he still washes his mowers with deisel, although now he gets me to replace the "way too expensive special mower belts"