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Probably the material of the roof shingles expands, perhaps as it heats or as it absorbs some moisture, causing the shingles, constrained transversely at their upper boundaries, to be in transverse compression, causing many of them to buckle upward.
FROM:
http://i.ytimg.com/vi/-UeOMw-gVII/hqdefault.jpg
This is a You-Tube video.
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