The cause is actually included in the article. “Wing stuck in mouth during embryonic development”. As for an update for treatment I mentioned that in another comment below.
Well written and informative article, although more pictures would be helpful. Also, many have never experienced water belly in their flock, so an overview of what it is is helpful.
Nice compilation! I do think that there are some errors (duplications, grammar) and that there are more Silkie breeders out there that I know of, but otherwise I think this is pretty good.