Like Astern mentioned, repetitive motion is the most common source of TOS but there are other sources. In my case, an MVA triggered my TOS. I wasn't able to use the arm/shoulder from day 1 after the car accident. But it took me months to get the doctors to notice the arm situation. Once I had surgery, things were much better until I injured the other shoulder. My artery is affected so surgery is in the near future.