Calves may be born with a number of limb deformities for a variety of reasons. The most common is called contracted tendons, which sounds like what you are describing. Contracted tendons is an inherited condition, but position in the uterus may also play a role. At birth the pastern and fetlock joints are flexed and cannot be fully extended. These calves often “knuckle-over” and walk on the front of their pastern and fetlock joints.
Many calves with contracted tendons recover without treatment. More severe cases may require splints, casts, or surgery, and the worst cases may not respond to treatment.