I discovered this a few days ago and i did some test in private server and results
Screenshot 1 - http://prntscr.com/sdi7tf i randomly opened fire before i take this screenshot and i also switched camera view 2-3 times.
Screenshot 2 - http://prntscr.com/sdi8q0 and player model moves by itself you can see the difference by looking at crosshair. I never moved while this was happening.
Edit: As far as i can see this happens as soon as you enter any server. Shooting or changing camera view doesn't cause this.