I agree - it's genetics. If the casque is deformed, then it's a calcium problem.
I've heard a few people claim that the shorter casqued veileds simply have some C.orientalis blood in them. Possible, I guess. I'm sure there's a lot of variety in the populations, so it's hard to say for sure.
one thing's for certain - veileds with large casques really show any problems with calcium - those things can be all over the place.
Edit - I agree, Brad. Sexual selection sure favors large casques in the wild. At least in Florida. Might be different in parts of Yemen, but I doubt it. When captiver beeding neglects to favor such things that would be a factor in th ewild, captive populations start to diverge. Probably no real benifit to the casque aside from the fact that females like it, it makes them look larger ( making territory defense more effecient), and simply aides in their ability to mate with more females.
It might not help the individual survive in the wild, but it sure as heck helps him get the girls.