Below is a link to a prior similar question with a couple of responses - sometimes just poor adhesion (particularly with paper tape that was not properly bedded in joint compound) is the cause, but generally moisture is the cause. If there is a more than hairline joint crack in the joint compound after the tape is peeled off, then movement of the ceiling structure is almost certainly the cause.
If there is no significant crack in the joint compound under the tape,, along the drywall sheet joint, and the tape peels off and has very poor or almost no adhesion to the drywall, or there is no sign of drywall compound under the tape, likely someone did not "mud" the joint with drywall compound before putting the tape on, so it was only stuck to the ceiling by the compound applied over it and adhering along the edges of the tape.
1) can be due to inadequate nailing letting the drywall settle - i.e. the fasteners pullingthrough the drywall, in which case you can push upon it and feel that it is not tight against the joists above, plus will likely cause nail/screw popouts when you do that
2) can be due to normal joist creep sagging over time (especially in long-span areas like open basements and garages) in which case the only joints opening up will normally be those running perpendicular to the joists above and normally only or more toward the center of the joist span then near the ends
3) can be due to overloading of the overlying floor or structural issues
4) on the moisture side, prolonged garage high humidity can do this (especially if not air conditioned and in humid locale), as can roof or pipe leaks or moisture from attic air conditioning duct from above, and in ceilings underlying an open-air (not "conditioned" space with heat and A/C) attic both roof leaks, condensation due to excess household moist air getting into the attic, and inadequate ventilation resulting in frosting or condensation in the framing and insulation can cause this
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