The thing that many people don't realise is that there is a very important ligament that attaches from the ball at the top of the thighbone into the hip socket. It is called the Ligamentum Teres or Round Ligament. It is important for the stability of the hip joint, but also carries a blood vessel that supplies blood to the ball part of the head of the femur (thigh bone). When you take the leg into a full side mount and then add on a side tilt, or take the leg behind your head, this ligament is taken in to a fully stretched position. If this is done forcefully, repeatedly, or without control, the risk of ripping this ligament is high.
If this ligament is ripped, surgeons don't tend to reconnect it. They may tidy up the ends but may not join the ends together. This means that the stability and blood supply to the head of the thigh bone (femur) may be reduced.
The reason why some people find the oversplit leg mount movement easier than others may be due to the possible anatomical vacation of not having this ligament. In this situation, it is even more important that the dancer learns how to carefully stabilise the hip using the deep small muscles that surround the hip to reduce wear and tear on the cartilage and avoid long term damage.