You know, you might not see it again...I just err on the side of caution and lower the protein. If it cropped up again, then I could rule out high protein. It's primarily cosmetic, but wrapping it really isn't too troublesome so I'll always keep an eye on those first flight feathers coming in.
Right - I think we may oversimplify in an effort to identify the cause (a single cause is certainly easier to understand and deal with). In reality, the cause could definitely be synergistic or multifactorial.
Like @dotknott said, angel wing can be caused by too much protein; I had one drake that started showing signs at about 8 weeks, but after a week of being wrapped in elastic bandaging (Vetrap), the problem corrected. I start my ducklings on 21% protein feed from hatching and cut the protein to...