G'day Jeff.

Jib and main were loose, but I forgot the downhaul (I've an very old sail, total out of shape, so maybe that is not an issue).
The main problem was that the wind was straight offshore behind high dunes. So that creates also down-wind squalls which press the sail on the sea.

The moment I flipped, I was just at the height were there is a high forestborder and there is with this wind a kind of venturi effect.
Just at that place I was so stupid to let 1 sec my mainsheet loose in order to adjust my traveller and just at that time there was this damned squall.

So total beginners-error! Shame on me, I always teach other people to never let go of the sheet; even if you have to itch or look behind you etc.