Sorry for late reply.

"Why only the shoulder"?.......good point. In my opinion, it's only the shoulder because the goal is only to restrict arm movement so it doesn't affect the neck. And, it's usually only one shoulder because that arm is the one causing the referred muscle reaction in the neck area. If there were other joints involved we would have to be looking at arthritic condition connections. Also, there is the question of inflammatory reactions in the shoulder leading to some temporary tissue damage. This might be explained by the cyclical pain eventually causing tissue inflammation and anything resulting from that. It doesn't mean that the inflammation or tissue damage caused the pain ! Everything returns to normal functionality fairly consistently within the same time frame for most patients, and that suggests to me that the cause was referred cyclical pain with a source other than the shoulder.