The A350s already have these doors. All the carts have these doors, not just the chillers. I've never seen any official documentation but I wonder if it has anything to do with safety and asthetics. Safety as a loose cart will not roll out of its stowage. And esthetically the galley looks a lot 'cleaner', especially at times when pax transit through the galley such as during boarding/deboarding, similar to kitchens with appliances behind cabinet doors.