My guess?
Engine stall or surge at full power. Lots of bangs and flames. Engines all retarded to idle. Stall/surge successfully resolved. But some muppet passenger who was probably talking loudly, or reading his broadsheet all through the safety briefing because he/she's an oh so frequent traveller, decided they'd seen enough to panic, and opened a door. After rummaging through the overhead locker first obviously!
I'll be amazed if that's not the scenario we're dealing with here!
Any passengers thinking about taking this sort of action in future. Do be aware that in a 'crew controlled' evacuation, the engines will be shut down (switched off) BEFORE you are COMMANDED to evacuate. The reason being that jet engines have a very real capacity to suck people right through the shredders and furnace before spitting what's left out the back.
Just do exactly as you are told by the professionals rather than panic. Don't turn a non-event into a death by your utter stupidity! And any IDIOT attempting to open an overhead locker during an evacuation down slides when I'm in the cabin is looking for BIG trouble!
WATCH THE BL00DY SAFETY VIDEO/DEMO FOR THE SAKE OF THE PEOPLE AROUND YOU EVEN IF YOU HAVE NO RESPECT FOR YOUR OWN LIFE!!!!!!!