This is a somewhat new phenomenon which has resulted from the changes in wood construction. Most new homes are being built from pre-fabricated roof and floor trusses instead of using conventional 2x rafters and floor joists. Trusses are comprised of 2x members connected together with metal plates pressed into the joints.

Wood shrinks and expands in response to the weather. In trusses, the changes in each individual member can have a net affect on the entire truss which causes the truss to lift. When this occurs large cracks may appear between the truss and the wall. This is necessary for the truss to act properly. However, it creates an unappealing aesthetic response. Assuming the trusses were designed properly, there is no structural significance from truss lift.

There are a couple of ways to accommodate this movement and minimize the visible cracks. Trim can be attached to the ceiling, extending down the wall, but not actually attached to the wall. In this way, when the truss moves the trim slides up and down which keeps the wall from cracking. Another method is to attach the ceiling drywall to the wall not the truss over the last few feet on either side of a wall. This results in the drywall flexing to accommodate the truss movement while minimizing and possibly eliminate cracking.