I guess as an unconscious adaption to hypermobility.

All of your joints, then, not just the major ones? I learned only last month that some of the things I can do with my arms are not normal. I've known for years that my elbows hyperextend—I had to learn to be careful of it when I did archery—but it would never have occurred to me that not everyone can reach up behind themselves and flatten their palms between their own shoulderblades. There was something else the physical therapist noticed after going through the full set of bends and rotations, but I don't remember what, just that she warned me about putting weight or torque on my body in ways it was not evolved to support.