It's safe to say that there was gasoline in the tank of the car. Cars can and do spontaneously catch fire, I've had more than one friend pull off the road and watch their car burn down. Fuel injectors can leak, foreign bodies can become lodged against the engine block, electrical systems can short out. There are reasons it's advisable to keep a fire extinguisher. A spontaneous car fire takes a while to really get going and under normal circumstances the occupants have ample opportunity to stop and get out but a person who is very intoxicated, less so.