Pointed cabbage

Had to think about this one - maybe aphid infestation (though I'd expect other signs such as the bugs themselves, yellowing and a pungent cabbagy smell), not enough nutrients especially nitrogen and soil possibly been too dry or growing conditions have been too hot would be high on my list.  The only other thing I could think of was possibly genetic (a natural mutant - sometimes happens and how we get all different types of varieties) but I don't think a whole batch would be the same.