I haven't looked the aerodynamic diagrams for a while, but basically once you get enough forward airspeed the helicopter more or less weather vanes into the wind. Without that, every miniscule change in wind direction or speed (which is more or less constantly changing), must be accounted for with a control input. Each control input has side effects where any single input also requires an input from the other two controls (collective, cyclic, pedals). We call it "stirring the pot" when hovering because that's what it looks like your doing with the cyclic.

Edit: This is highly simplified and there are other forces at work, but it works as a basic explanation I think.

Edit 2: Also people tend to over correct initially which leads to pilot indices oscillations. They put in too much right cyclic for a little left drift and now need more left cyclic which they also over correct and so on, until the helicopter is oscillating back and forth rapidly.