This is becoming quite common now - noticed the change myself recently on QF (was both announced and enforced) and BA have had such a policy (often quite vigorously enforced) for a few years.
Rationale given is that its a safety policy so that you can hear all announcements in an emergency, and have nothing that can impeded your quick evacuation of the aircraft (you know, like a headphone cord, plugged into the seat socket, wrapping around your neck as you rush from your seat!).