Vitamin b deficiency. I was getting these until I realized that the medication in chick food was a thiamin blocker, and I was getting them no matter who or what incubated the chicks. (We gave some eggs to others to incubate for us, from many breeds and incubator types.)
Anyways, as well as medicating food, a thiamin blocker blocks vitamin B, and lack of vitamin B gives curled toes. Once I started feeding unmedicated, the new generations never had any, but you can't really reverse curled toes.
Curled Toes can be reversed, the first CTP I treated took 2wks, his toes straightened out & he was walking. Small as he was I had to keep him separated from his flock of hatchlings, would go after the bigger ones. When he went back with the rest to their owner, he got more spoiled