I can only speak for BA.

At 20 minutes to landing, the seatbelts signs are switched on.
CC need to secure the cabin in preparation for landing, which incudes collecting in any used headphones. (the leads are loose and are therefore a hazard). Preps also include restowing of screens. Obviously in lots of seats, i.e. non front row WT and WT+ seats you can still see your screen, but it is still switched off.

Another reason it may all be switched off is due to power maybe?