Why did my Ford Escape 01 burst into flames while it was being towed?

The Backround information, my vehicle started to slow down and like couldnt build speed. I was only going 40 and running at about 4000rpm so I pulled into the TA that was just about 200 feet down the street and it turned off as I pulled in. So I let it sit for a few mins turned it back on and parked out of the way in the truck stop. Then I called my insurance company and they sent a tow truck. So we are go about 100 feet and the driver see my car trying to turn, he forgot to lock the steering wheel, so he fixes it and everything is fine for the next 16 miles, and all of a sudden my car bursts into flames. The engine wasnt on but the key was in it as the car was in neutral. The key was partially turned(dont know why he left it that way, prolly to stop the alarm) but that shouldnt make the car burst into flames. We are both fine, we were able to disconnect and get away from the car. Car is totaled.
Added (1). It was automatic, uh two of the wheels were on the ground and it was in neutral with parking break off

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