When you shoot a weapon, your upper body recoils, and the way it does it just seems unnatural. This is observed most when you are in a prone or low prone animation. A video is below to show what it looks like.
It just overall is too violent and unnatural looking. Just an FYI, the recoil IMO is too violent also. All of the weapons do it, I noticed the MX series of rifles does it the most along with the Mk200.