With both cowls missing from both engines, it looks like there's a strong possibilty that the fan cowls weren't latched properly/if at all.
I would think it highly unlikely that the engine fire bottles blew the cowls as only the right engine looked like it had a fire issue which 'could' have been caused by the cowl damaging fuel/oil lines as it depated that engine.
It will be interesting to see if the engines had maintenance caried out on them prior to this flight.
Obviously this is all purely speculation and my own views, but from the photos/footage I've seen it's an educated guess.
We'll have to wait for more information and investigtion to confirm or otherwise.
Either way, great job by the crew and all involved.