The femoral head surface is covered with a smooth articular cartilage.
Muscles encase the entire hip, stablizing and allowing movement.
The head of the femur is held in the acetabulum by the pelvic muscles, joint capsule, surface tension and the round ligament. Proper development of the joint still depends on the head of the femur being held firmly within the acetabulum until all parts are mature. Sometime after birth, something initiates a bad fit or function of one or more parts. It is likely that there are multiple causing factors and they may fidder between genetic lines. Any attempt to define the process in exact sequence is speculative.