There have been studies done of wild Canadian geese as the angel wing rarely (if ever) occurs in the wild. One season they free-fed gamebird feed to a control flock in the wild - lo and behold, angel wing appeared in the goslings. It is caused by free-feeding during the rapid growth stage of goslings (not sure if the same applies to chickens). The higher the protein, the more common the problem.
Now that I know the cause, I am able to prevent it. But it still tries to occur in my sebastopols. I think this is because their long feathers make the wings even heavier. In geese, angel wing is not a genetic disorder.