possibly the drywallers did not screw the corners in with enough screws or forgot them all together.
depending on how much is cracking off them, you could scrape the drywall compound off and see if that is the reason, then add more screws and re-apply compound.(the small holes get the screws about every 6 – 10 inches.)
but you may have to get them to come back (if you have the new home warranty in effect there) .