It looks like Treyarch has been watching a little too much Doctor Who, because when you shoot off the arms of all the mannequins on Nuketown this happens:
The mannequins become just like the weeping angels in Doctor Who, only moving when no one is looking at them. Though, when they do move, they move fast and they will claw up your back something nasty.
It's odd to praise the animation on figures you only see in states of frozen motion, but Treyarch's done an awesome job on the way the mannequins move. They seem to be flinging themselves at you each time you turn your back.
T.O.P. over on NeoGAF shared the secret and it was quickly pointed out this isn't the first time Trearch has slipped an Easter Egg into Call of Duty's Nuketown map: if you shot off the heads of all the Mannequins in Black Ops 1 you were rewarded with The Rolling Stones' Sympathy for the Devil playing over the map's speakers.