I'd say muscle tightness in the neck causes TOS. The nerves get compressed by the scalenes and possibly other muscles which is what causes symptoms in shoulders, arms, hands etc.
It is also possible that compressed nerves in turn cause muscle spasms in different places.
Prior to getting full-blown TOS symptoms I was getting bad spasms in the back of the neck. Don't know if TOS was the cause or the result of that though.