We're guessing the insurance company will declare the car a total loss

They can be the result of poor vehicle maintenance, or the result of design oversights:

http://www.mobiloil.com/USA-English/MotorOil/Car_C...

http://www.cnn.com/2005/US/06/16/ford.vehicles/

It is also safe to say, no cars are safe from vehicle fires:
http://jalopnik.com/5937499/the-jalopnik-guide-to-...

I even heard of someone melting the front of their Rolls Royce once. They were attempting to jump start it with some cables, and well, let's just say that car never started again.

• Fuel or brake fluid leak onto hot engine components, typically onto the exhaust manifold from a compromised fuel line or brake line. Oil seeping onto the exhaust manifold from a compromised gasket (often the valve cover) or from someone who put oil in the car but forgot to put the cap back on is another one.
• Combustible material contacting the catalytic converter, often when the vehicle is parked on top of dry grass/brush/leaves. Much less common these days now that air/fuel mixture is better regulated, which means the converters have less of a tendency to run hot.
• Another common one on old cars was backfires through the air cleaner catching the paper air cleaner element on fire.
• Electrical--some Fords were recalled in, IIRC, the mid 90s because the ignition switches tended to catch fire. The kicker? They'd do this when the ignition was shut off and the key removed, often with the car parked in the owner's garage. There are also a lot of people who do inexpert accessory installations, commonly aftermarket car stereos, auxiliary lighting, and so on.
• Overheated brakes. Commonly, a stuck caliper means the pads don't retract fully on one wheel when the brakes are released. Eventually, the pads on that wheel overheat to the point where they catch the pad material, the axle grease, and the brake fluid on fire.
• Similarly, other overheated stuff...bearings or sliding surfaces that run dry of lubricant will eventually overheat.
• Careless disposal of smoking materials inside the vehicle...dropped cigarette butt/roach lights old fast-food wrappers, toll receipts, carpet, etc. on fire
• Arson. Might be obvious, but has to be said. A lot of car thieves torch the vehicle in hopes of covering up the theft. Sometimes the owner can't keep up with the payments.