The actual cause of frozen shoulder it not fully known. This is probably because people get frozen shoulder for so many different reasons. But by and far the number one way that I have seen people get frozen shoulder is when they stop moving shoulder for an extended peroid of time.
Common reasons for doing this are because your should is in a sling, perhaps you have injured your shoulder, or you might be recovering from surgery. When you stop moving your shoulder joint, things called adhesions form and prevent your from moving your shoulder through a normal range of motion. So it is sort of a self fulfilling prophecy. Stop moving your shoulder and adhesions form so that you can’t move your shoulder.
Watch the video below to learn more and hear my entire answer.