You know the idea that we should keep our shoulders "back and down"? Like all the time, in every yoga pose that we do? Well our shoulders were actually designed to *move* - not be pinned into one position all the time.
There are four (four!) joints that make up the shoulder joint complex, and regular movement keeps all of them healthy. Lack of movement, on the other hand, just makes your shoulder joints mad at you.
Why would we want to train our shoulders not to move by keeping them pinned back and down all the time?