Many people, as well as I, have broken boards in a similiar fashion.
Wouter once had a post, I'll try and summerize some of the points I took away from it that I now use in my sailing.
1. At high speeds, you don't need your boards all the way down. It in fact can slow you down some. With it up slightly, it really changes the stress point on the board, making it much less likely to break to windward.
2. If trying to point max as in going to A mark, in light to moderate air, then all the way down is good. But when your hull speed is great, slide the board up some, it'll handle better, and less likely to break the board.
3. Flame suit on
my 2 cents
Todd Bouton
N6.0na +
#111
several non inpact board failures in the past, none since adopting the above theories