This can be caused by pelvic asymmetry, decreased flexibility in the ankle or hip, and lumbosacral dysfunction. Try sitting at work or school all day with poor posture, crossed legs, driving in the car, lifting incorrectly, I could go on and on. This is not a problem you can fix on horseback. Asymmetry needs to be addressed before the rider gets on the horse. Unless you are a licensed physical therapist or physician with the ability and time to properly evaluate each riders' structure, you as a trainer have a responsibility to point the rider in the right direction.
Riders come to lessons with a lot of baggage including old injuries.
Riding is a symmetrical sport no matter what the discipline. If you are going to pound anything into your students' head, pound in the fact that if they are crooked they will not achieve a balanced seat and their horse will pay for it physically. Many people do not have any idea what the spine looks like and how it operates. Ignorance of the structure of the spine is not acceptable in riding. The spine is the core of the rider's control and should be treated with great respect i.e. good posture, lifting correctly, and regular exercise.