It's referred to as corner bead or corner aid out here. There can be many reasons, but the culprit is the edging pulling loose and knocking mud off. I would get some ring shanked drywall nails and tighten up the perimeters of the metal. You might as well bite the bullet and tighten them up top to bottom, or they will keep reoccuring. Patch, primer, and paint.

Thanks! What causes it to pull loose? All I can think of is the house contracting and expanding seasonally, which is not likely to stop.

How would I do thisÂtell me if this is right. Scrape/chip off the mud down to the metal where I want to nail, drill first, then nail, then do the patch/primer/paint. Or drill and nail right through the existing mud? How far aapart for the new nails? Thanks!

If the mud pulls off fairly easily, then go ahead and get off what you can and just nail through the rest that wont come off. After nailing, it will crack the mud in those areas where you can pull it off then. You dont necessarily have to drill the metal as it is thin gauge and drywall nails have a sharp point that easily punctures the metal, but good idea of yours to have a drill / bit handy in case. I would nail every six inches and you might have to nail closer in trouble areas.

As to causes....settling, slight warping or movement of the corner framing, drastic humidity changes in the house, sometimes even if the joint compound was thinned making it weak initially,are some of the causes.