Those are the reverse thrust buckets. Airbus is different to Boeing in that they have 'doors' that pop open from the side of the engine and divert the flow forward. Boeing's have a moving section of cowling that slides backwards.
As to why some aren't open, as the aircraft has just touched down, i'd hazard a guess they were in the process of opening - you can see this from the No 4 engine bottom scoop being only half open.
Horgy