But why? One theory is that as the wind is forced up and over the crest of the roof a vortex is formed and the wind curls down and strikes the leeward side lower down. But having clung to a roof during high winds - don't ask (well, if you must it's because I'm a householder who regularly keeps a spare set of roofing crochets in his pocket and likes to rush out during lulls in the driving rain to repair what he can before the next deluge) - and I can attest that the leeward side of a roof, vortex or no vortices, is a vastly preferable side to cling to than the windward.
So, a new theory is required. And, I think I may have it.
Slate suicide. It may sound strange but think about it. In Normandy the leeward side faces north. What more depressing side is there to look out on? Barely any sun, cold northerly winds, the last part of the roof to lose its snow. The slates have to suffer from S.A.D. (Seasonally Affected Disorder or, as it's known in Scandinavia, why Swedes Are Depressed)
So the depressed slate - no doubt pining for the fjords or some Welsh slate mine near Blaenau Ffestiniog - decides that shuffling off its mortal roofing crochet is preferable to sitting though another winter of storm and frost.
Hmmm, maybe I should apply for an EU research grant? I have enough roofs to set up a control. I could play uplifting music to one northern face and Gloomy Sunday to another.
Or, maybe, I could take my medication.