And probably too when we're in pain or even just discomfort - we tend to brace ourselves subconsciously.
I think the whole body fascia thing comes into play too, like Sharon Butler explains in her book.

My generic theory LOL

Muscles get tight or spasmed from the loss of blood flow and /or messed up nerve signals and that causes loss of the fluids, circulation, oxygen and cell nutrition.
Those conditions sets it up for the next muscles to join in and so on with neighboring muscles trying to brace the injured ones. What ever affects the muscles that way I'm sure will affect the fascia too and it just spreads- not actual TOS per se but the similar fascia sx.

I done sprained my brain trying to get that to make sense LOL
must be time for mid afternoon coffee

I think much of it does come down to postures/alignment, at least for myself, I can tell when I have been lax on those self care things and need to get serious about it again.