Goomba: Those doors are called "raised Panel" doors. The centre panels have expanded and pushed the joint apart. When cabinet makers build doors like that they make sure that the small tongue around the panel is undersized in relation to the groove that it fits into, to allow for expansion and contraction. Problem is they can also rattle when too loose. A woodworking trick is to place small rubber strips into the grooves (after relieving the edges of the tongue's) that will allow for the swelling or contraction and still place a little pressure on the panel. Check with a local cabinet shop. they should be able to make then "like new" again.