Poor model rigging causes that.
As specific parts of the model are moved by specific skeleton bones, anything in between gets stretched and compressed as you move. If you aren't careful then you can get some funky results. Me thinks this is just a little poor riggin on Obsidians part for the defult body mesh, though really, how often are you running around without armor?
It's not just without armor, you can greatly see this issue with armor on, and as I player who mostly plays in third person, aside from combat, it is kinda annoying. Like I said, it looked fine up until last night. I stopped to eat some food, started the game back up, starting seeing this "bug". Is there any way to fix it?
To be a little more clearer, it's not just the shoulder, there some issues with the hands as well, even on males, all models, really.