Either the underpin in that location has failed or the door and frame are swelling. Since we have had little rain, I would tend to think that the underpin had failed over time and has now lost the uplift capacity to keep the door frame in place.
It would be wise to install a crack meter at the door frame to watch over the next 30 days to determine what exactly is the problem.
These are obtained from wholesale companies that specialize in testing equipment, I'm not sure if they sell indivual packages. If you really need one then try to find a company online that sells them individually or you can give us a call and we can place one on your property for you since we purchase them in bulk.