Not real sure what happened. Do know the mower is about 11 years old and mowed about 4 acres a week during mowing season. There was a small oil leak under the motor. It is serviced by the other people in the maintenance dept. I could see oil on the motor pulley after they brought it back to me from its yearly servicing go figure. The way these tractors are made too may have played a factor as the exhaust comes out the front. It could have been some leaves blown back into the motor as it was windy and very dry. I was watching the fence line mowing not to get caught up in it with the deck, tire chains, etc. Next thing i saw were flames coming out from under it. I didnt hesitate to get off of it as I just filled it up with gas. It was a matter of seconds after i got off that the whole thing was engulfed after the gas tank melted releasing gas. Not much i could do after I called the fire dept so i snapped a couple of pics. It was a fairly decent mower, troy-bilt 22hp but it was ran to death though.