Remember, plastic melts. Easily.
For the record, this BMW i3 didn't catch fire on its own unlike, say, the Fisker Karma. Nor was its charging unit to blame. This i3 just happened to be at the wrong place at the wrong time. That place? Its owner's garage. BMW Blog recently got a hold of these images showing the nasty aftermath of a garage fire that was apparently started by still smoldering firework ashes in a nearby trash can. Firefighters managed to put the blaze out but not before the flames melted the i3. Literally.
That's what happens to a car that's composed mainly of Carbon Fiber Reinforced Plastic (CFRP). Just compare what's left of the i3 to the also charred 328i parked next to it; the latter's body still exists. Metal doesn't melt quite so easily. The owner even took a couple of photos of the garage's cement floor, now with CRFP resin. Though it's hard to tell from these images, only the i3's aluminum frame and battery tray were still intact. Photos courtesy of BMW Blog.