As was said earlier, the Airbus door (if armed) will disarm if opened from the outside. Naturally, if opened from the inside, the slide will blow (as one would hope anyway!).
One problem that the Airbus does have (maybe Boeing's do too, not sure) is that it's very prone to getting debris stuck beneath the door, sugar sachets, cutlery, security seals etc. etc. Its not uncommon for a door to be 'armed', yet in practise for the girt bar not to have engaged and thus show as disarmed on the crew's FAP and on ECAM.
Just a theory, perhaps the door was disarmed after flight, but ina reversal to the usual scenario, perhaps something was stuck so when the door was opened, it tugged the girt bar and deployed the slide??
I don't know the facts so please accept this is just a thought!!!
FJ2k