Mechanics go bad at hand break. Need to pronate( turn both thumbs downward and point both elbows toward up) . This forces the humeral head into the socket. When the lead foot turns over the throwing hand will come into the window naturally.
Try this ...Keep the lead thumb (in glove) pointing toward down and try to open the front shoulder. Now get in the same psoition and rotate the glove up and feel the shoulder release.
Now you know what causes the lead shoulder to fly.
Teacherman is correct in his statement IMO