They can injure their feet while cracking out of their eggs, or sometimes they have nutritional deficiencies that cause this problem. Uncurling the feet helps build up the muscles that control their digits, iirc.
From what I remember, it's not too uncommon across most birds. Sometimes, when you see a curled foot, it's simply because a bird got in a fight or landed wrong and hurt their foot. It also happens when birds are raised in improper spaces with hard flat flooring.