The whole point to gas is the wide set of "specific circumstances" in which it is flammable. It is most flammable in its gaseous form so any fuel tank leak at all during the accident could have spilled gas all around the car which (due to its volatility) would quickly flash into a vapor form.

At that point it needs only to find 1 hot-enough part of the car to ignite and spread throughout the vapor cloud to start burning all around the car.