Well it could be several different things, first off it could be a deformity, how long has it had this problem? Also could of got it's back legs caught on something a twisted backwards, but I really don't think that’s the case, because it's both of the legs and there would be some swelling/bruising. Also it could be a calcium deficiency(M.B.D.) but you said it was a hatchling and it is probably way too young... Some more information would be very helpful, also pictures would be great! And has it ate anything? Does it drag its legs behind him when he walks? Does he seem to try to avoid walking on his back legs, like he was in pain?