he probably simply over charged it with the wrong charger...Li-Po batteries don't like being over charged
That and/or buying cheap, counterfeit batteries from China. Those are OK for perhaps a flashlight, but not a high-current application like a vaporizer.
The button was pressed for a long time when he put it in his pocket.
All of the ones I've ever used will shut-off after 3-4 seconds as a safety precaution against that very thing. I can't vouch for this poor bloke's brand though.
Something similar happened a couple of days ago in the US. A lawyer was in court when his e-cigarette set fire to his trouser pocket. The best thing was, it was an arson case...
I love the guy that comes over, fuck the guy whose trousers just exploded, he just wants to make sure his Mini is alright. Now that's what you call a Mini adventure ...