In order to join two sheets of drywall together tape must be placed over the seam first before applying the joint compound or "mud". This will reinforce the joints and allows the "mud" to do it's work. So what other reason than poor craftsmanship could cause the tape to separate from the drywall?
The answer - MOISTURE! Whether it is a direct leak or just excessive moisture in the air the tape will come loose and pull away from the drywall. Before you call your contractor with complaints of shoddy workmanship take a look at what else is going on in the room the separating is occurring.