Hi Farid,
A/THR can be disconnected for several reasons. It could be playing up or you have a problem with it, it may not be coping well with the conditions (i.e. turbulence) and the pilot may wish the keep the engines at a set thrust instead of chasing a speed.
It may just be that when you have seen it disconnected, the pilot was practicing a manual thrust approach. Airlines have different policies on this...
To Disconnect, as you say, select thrust idle at touch down. If you are doing the manual thrust approach, it is important to ensure you bring the maximum thrust back to what the current N1 is on the display. If you don't you risk the aircraft either entering THR LOCK, where the thrust doesn't change, or worse you disconnect using the instinctive disconnect button on the side of the thrust levers and the N1 shoots up to climb thrust and you make a meal of your approach.
I hope this helps.
(This post is not intended to teah anyone to fly or arguing Airbus operational philosophy, just a bit of info I have picked up.)