short version -as I understand it.
And logically it makes sense to me.

The more forward your head and shoulders go over time - can cause the muscles to shorten in the front of the body.
This can crowd the top rib/clavicle space affecting the vascular/nerves and also strains the scalenes - especially if you don't do activities/postures to counteract the forward head/shoulder postures.

Also the muscles and tissues in the back of the upper body get stretched and weakened. Then they can't hold you in proper alignment anymore and will develop spasms & trigger points.

Another thing is the static postures at a desk/computer and some other jobs - where the movements are small and limited and does not get the blood pumping to keep tissues oxygenated.

Then there are possible preexisting factors -
hyper mobility
previous MVA or whiplash
large ligaments or other bony anomalies