No idea. Switch mode PSUs, of which this was one, all have so many potential modes of failure that some of them are bound to be catastrophic and dangerous. My money's on a capacitor shorting out and a safety cutout failing to cut out, something like that. BIt worrying though. Makes me think I need to revisit my policy of always keeping wall warts and other external PSUs on ceramic tiles to reduce the chances of them catching other stuff (floors, carpets, furniture:) alight. I used to do that but I stopped. Time to start again maybe.