What causes a car to randomly catch fire?

So I was on the highway on my way home from work when I noticed a car on fire. It wasn't in a accident or hit anything just parked on the side of the road while the owners and the police watched helplessly burn. It kind of made me sad thinking what if that were me. Then I started freaking out thinking my car could do that. The car that was on fire was a newer Ford Explorer (like 2012-15 ish). That really made me freak out because I drive a 2014 Ford Focus. I know it could have been a lot of things but it made me terrified of my car for some reason (which is fine and not on fire).

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