That seems to be the most common, but I know of at least two other scenarios:

-- Some young people in their teens get TOS, possibly from anatomy (such as cervical ribs) or posture or repetition in sports or injury, or some combo thereof. One guy developed it before he was finished growing and by the time it was diagnosed and corrected, he had finished growing. Consequently, one of his hands is smaller than the other. He posted on youtube about it.

-- Injuries, such as a broken collar bone, can leave someone with compression in the thoracic area.

There may be other causes. That's just off the top of my head.