Perhaps someone needs to go check all similarly-configured 777s with enough flight time to see if it's some form of metal fatigue due to the airflow past that region causing pressure fluctuations and oscillation of the panel.
I assume the thrust reversers don't cause a pressure wave around there? That could cause it to go pop on deceleration. Some poor sod will probably have to sit and listen to the CVR and see if they can hear when it happened.