> other cars don't generally spontaneously combust

They do. Check out YouTube, or the anecdotes in the other comments here. I'll throw in another -- one of my neighbors' cars spontaneously caught fire sitting in his driveway after midnight. The fire department had to come put it out, along with the front of his house it started to set fire to.

Car drove by my house way back when, smoking rather alarmingly. Stopped a few dozen meters further on, fire from under the hood, firefighters dropping by to extinguish it. Adventures in one's youth. No clue what caused it; probably a short circuit, and given how there's oil and gasoline everywhere, it kinda keeps going.