I had an online exchange with someone who had that happen. He said there's some kind of radiator on the side that will sometimes get chaff built up in it. That buildup will occasionally get hot enough to ignite, and, in their infinite wisdom, John Deere put it right over a plastic fuel tank...
Sounds like the hydraulic cooler. But those things throw so much crap in the air that if the wind blows right everything gets crud in it.