Buckling shingles can become humped or wavy, usually running in a straight line up, or across the roof. Possible Causes:
Deck or felt movement often caused by varying moisture levels in the material under the shingles. The shift will cause the nails to move in accordance with the deck forcing the shingles closer or farther apart depending on the situation.
Improper nailing - nails will 'pop up' creating the buckling effect.
Shingles nailed too tightly together.
Thin decking (3/8”) over 24” centers can be the cause due to clips, meant to hold the decking in place, coming lose or coming off.
Depending on the cause of the buckling, improving attic ventilation to eliminate excess humidity, removing fasteners and refastening or replacing distorted shingles can be some possible solutions.
- http://www.dreamroofs.com/en/02/p_02_061.aspx?id=16 -