My assumption is a very minor fuel leak, and that wasn't noticed, turning into a more significant leak, during an emissions burn; but regardless, you can't say yes or no, and even if you could, you wouldn't be able to prove it's a factory defect.
There isn't a chance of vandalism, is there? Do you leave machine onsite? It wouldn't be the first time a mad neighbor or "concerned citizen" tampered with equipment.
Heck, it could be anything, and unfortunately fires happen, but what I've seen 95% of the time, it's either leaves/debris, or feeding a burn pile. Doesn't take much to start a fire with some leaves or pine needles in an oily/fuel area; or a couple old oily shop rags. Normally general vandalism isn't very sophisticated; although I've heard of a unpaid subcontractor putting lacquer thinner in fuel tanks, and allegedly leading to engine problems (overheating? maybe leaking past rings and stripping oil, basically making it like a no oil run? maybe it's 100% myth?)