Slight curves (not being perfectly straight) can be normal. I have seen fresh WC veields with slight imperfections.
Major curves are usually from MBD.
Most cases of MBD are minor, and don't even result in permanant deformity. Some cases result in things like the above casque, broken bones, etc.
I have a beautiful blue male veiled, from german bloodlines, that arrived with a broken arm and VERY warped casque. He's fine, but the MBD ha sleft his mark.
It's not JUST due to improper supplementation, UVB and gutloading. Artificially fast growth rates, induced by excessive feeding at a very young age, will increase the odds of MBD. Many commercial breeders feed them so much, and grow them so fast, minor MBD is actually a sure thing.
I had two females develop severe MBD at around 3 months of age despite being given unfiltered sunlight. They were overfed as babies by the breeder.