As Ken mentioned, car fires can spread to other cars or structures.
A fire in the engine compartment will generate significant amounts of heat. Many of the materials use in the interior of a car will generate combustible gas fumes and melted liquids when heated to temperatures x. y or z, especially the vinyls/plastics used for seat covers, carpeting, etc. The interior of the car acts like an oven from the heat in the engine compartment, building up heat that generates more volatile fumes. The fumes continue to build up and heat up. At some point, "something" happens that causes a flash over or rapid combustion occurs in the passenger compartment. A window cracks or a window seal melts and allows enough oxygen in for "poof".
When the gasoline in a fuel tanks leaks out and catches fire, it burns at over 1500ºF. The rubber in car tires ignites at 750ºF, so after a few minutes of radiant heating from a car fire in an adjacent paring space, the tires will catch fire. Now the heat from THAT car's tire will start the burning process in it's car. Radiant heat will also go through a car's windows like the sun's rays in a green house. Cars can go up as if in a slow motion domino cascade. Cooling down a single car in the chain will stop the spread (assuming no other heat inputs - trees, grass, etc).
Vehicles set grass fires all the time. A catalytic converter in a vehicle's exhaust system gets extremely hot and will torch off any dry grass under the car. Mufflers can spark fires as well. So always watch where you park in the boonies. The car you save might be your own.
The Texas Forestry Service recommends that people keep brush and grass cleared back at least 30 feet to prevent house ignition from radiant heating. Wood ricks should NOT be kept adjacent to your house. This wood is usually VERY dry and ignites easily. They can act like a wick to starting your house to burn. Extremely dry wood, such as that on older houses, can start burning at temperatures as low as 360ºF, but only after significant exposure times, where the wood has a chance to heat up. At temps over 800ºF, ignition can occur in as little as a minute, depending on moisture content, flammability of any paint, etc.
People dislike it when the fire department tells them they need to "cut down nature" around structures. But then they get all bitchy when that "burning beautiful nature" is threatening their homes and we tell them "Sorry, we can't save your home. It's just gonna go."