I'm not a fireman, and I'm not in the US either. Most of these things I've learned from friends (firemen, policemen, doctors, and others also), and family (my father, as a doctor, was doing EMT service, and my sister is presently out there in an ambulance).

As I know, most -cumulative- vehicle fires started in the engine compartment. Most people are curious about the fumes under the bonnet, even after a smaller crash. And opening the bonnet for checking the case they are simply instantly burned by the freshly fed flames.