It is not uncommon for a portion of the body to move in the opposite direction while attempting to apply force to move an object.
High level hitters often have their front hip rotating rearward as the barrel is swung forward.
It's perfectly normal to have a body segment moving in the opposite direction in which you are trying to move an object.
Notice here that the barrel is moving in the opposite direction as the young girl's head.
I have no issue with the head moving back as her body moves forward. It's the severity of the way it snaps downward. As somebody else in the thread mentioned, it looks like something could snap at any moment. I'm not a doctor either, , it's just that movement looks a lot more severe than it needs to be.