Cor - check this out. This is something that occurs reasonably regularly...en. In this case there's a surface shear caused by friction of the wind against the ground - yep, good old-fashioned wind gradient. The wind is blowing...
See the article related to this amazing graph describing why maintenance needs to be planned and cross checked carefully!!!:
http://alltrikes.com/elgg/pages/view/40208/aircraft-maintenance-a-necessary-evil-waddington-efffect-opinion-mike-busch-a-well-known-apia-and-owner-of-savvy-aviator