Explain this drywall crack?

A gable-end wall of my 1-story house forms the wall of two approx. 14' wide bedrooms; that wall in each bedroom has two 14' sheets of drywall applied horizontally (using screws, naturally). The studding is 2x6's. A hairline crack at the horizontal seam has appeared in both rooms. The seams were taped with fiberglass tape and mudded in the usual fashion.
I can't think of a mechanism which would cause a horizontal seam to open-it would have to require extension of the 2x6 studs, wouldn't it? I can also see it happening if the studs could bow inward sufficiently, but they're pretty stiff.
I think it may somehow be related to the fact that this wall faces the main wind direction (westerly), and the gable does not provide as much stiffness as a loadbearing wall on which the trusses sit--the wall may flex from windload?
Any ideas? No other wall has done this, but the other gable is in the garage and I used vertical sheets there because it was easier to do--also it is comparatively sheltered from wind. Besides, why would you get cracking in the garage where nobody cares?
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Do you have anything rigid all along behind the horizontal seam to back it up. If not, the two pieces may vibrate or move at different rates (ie, upper flexes out when lower flexes in and vice versa). So they may not be moving apart, just differently.
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The drywall shrunk?
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Sounds probable. The first year we were in the house, the wall actually moved under wind pressure. I went up in the crawl space and put a stringer across the trusses and tied it to the top of the wall. This stopped audible motion, and possibly if I now worked on the crack again it might stay fixed. But that wall is always going to get some vibration and stress from the wind, and no, there is nothing in back of the seam.
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