My informed guess: Non-pyrotechnic internal-origination pressure event that blew the cabin skin and then removed the fairing like a champagne cork.
Could be "normal" pressure and a corrosion-weakened skin - or an overpressure source (O2 tank/line, luggage item that popped). Nothing in the pictures yet rules out either, so this must await examination of detailed evidence.
I notice that the Beeb is at last starting to get the msg on "plunge" - a graphic, and an aviation expert's quote, explaining that the descent was standard emergency procedure, now grace their web site.