Lithium-ion Batteries can & WILL become volatile in extreme conditions
I bet you money he didn't just drop his keys, but the phone too...and put it in his pocket after fetching it. The Li-ion batteries can be sensitive to shock.
Google : "A Lesson In Lithium-Ion Volatility -- Don't Try This at Home"
We seem to forget the energy density in these new batteries is very significant. I'm an avid model aircraft flier and the move to Li-Po battery packs for electric planes has offered me two instances of the battery pack going 'thermonuclear'.
Once, in a rapid charging station undergoing a typical cycle, after the pack had fallen 3-4 ft to concrete. The pack blew up like a balloon, developed a blister that rapidly burst into flames jetting out. The other instance was after a crash of an F18, where there was a sudden pop about half a minute after it hit the field. A flame bust up about a foot off the ground with a nasty hiss. What was left of the canopy was a fused mess of EDF plastic.