Why are recently repaired drywall joints in my ceiling bulging?
USG calls this defect joint ridging and offered the following cause and remedy.

Cause: All building materials grow or shrink in response to changes in temperature and humidity. When they are confined to a specific space, such as gypsum panels in a partition or ceiling, they are put under stress, either compression or tension, depending on the temperature or humidity conditions. These stresses are relieved when the panel bends outward in the region of the joint. Once this bending takes place, the system takes a set and never returns to normal. It becomes pro- gressively worse with each change of temperature or humidity. This progressive deformation appears as a continuous ridge along the length of joint, with a uniform fine, ridge-like pattern at the center.


Remedy: (1) Let ridge develop fully before undertaking repairs; usually six months is sufficient. Make repairs under average room conditions; (2) Smooth ridge down to reinforcing tape without cutting through tape. Fill concave areas on either side of ridge with light fill of compound. After this is dry, float very thin film of compound over entire area; (3) Examine area with strong sidelighting to make certain that ridge has been concealed. If not, use additional feathering coats of compound. Redecorate. Ridging can recur, but is usually less severe. Continuous wetting will aggravate condition.