This is an old thread but I didn't see any comments regarding a shoulder impingement which can prevent the body from working in the proper motion. If she is coming up the front side of the circle in a handshake position and maintaining that handshake position without letting the shoulder complex to rotate thru to the backside she will have to stick her butt out, hop sideways, in order to get her arm into a semi-coherent throwing position. That limiting of shoulder motion binds the body mechanics instead of letting them flow freely.
Visualize a tire with a with a serious flat spot if that helps. Thump, thump, thump. Shoulders are supposed to be smooth. Bad things happen otherwise.
Probably because they haven't hurt themselves (yet) and no one has caught it, or realizes what they are seeing... Mine would probably still have it if Java hadn't pointed it out in a private video and hers was mild compared to some I see.
And you're right. It took some serious PT to correct.