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.
http://www.youtube.com/watch?v=GMUE3JFjyT4&feature=youtu.be
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.
I think movement like this is indeed too violent, although maybe this could be retained for firing larger Calibre Weapons like 0.50cal Sniper Rifles.
http://www.youtube.com/watch?feature=player_detailpage&v=_9qMxjUjEJA#t=40s
It probably made sense while the game was still in the phase of low recoil weapons development, the issue really is that the system has not scaled well to the heavy recoiling weapons. Hopefully this will be tweaked to be more reasonable in appearance.
As of beta, watching people shoot beltfeds and sniper rifles is like watching a deranged human accordion :)
Thats the way it works in the engine. Maybe someday we will make recoil by animation, but until then it stays like this.