10 .The eagle-eyed might also notice that the flaps are not set (down), i.e. for the passengers to slide down, if they're evacuating via the over-wing exits. That would most certainly be due to a lack of hydraulic power; what with the engines having been shutdown and / or the electric hydraulic pumps (which would be slow to run in any case) having been switched off (as part of the normal on-chocks shutdown sequence) and which the flight crew had probably completed, i.e. prior to being told about a 'fire'.
I am guessing you're from the boeings... the a320 has overwing slides unlike the boeings
