This may be a congenital defect of the nerves, muscles, or bones; an injury that occurred at, or shortly after birth; or possibly, white muscle disease, a nutritional deficiency of selenium. From your description, it appears that if it is a defect or injury, the lesion is in the hips or lower back. Injuries like this might happen during an assisted delivery, or if the calf was stepped on by a cow. What matters most is whether or not the calf can get better and what you might need to do to help it.
A veterinary examination would help to determine the nature of the problem and what to do about it. If the problem is white muscle disease then injections of selenium (with veterinary supervision) might help, and knowing so would be important because the same condition may be affecting other calves.
