I believe that most of the time the deer are not purposely "ducking the arrow" but rather dropping down as they gather their legs under them to turn and run. It stands to reason that Kickin_Buck is on the right track with an alert Deer being more prone to "jumping the string". I tend to believe they are reacting to the sight or sound of a bow being released than an arrow flying at them. One last thought, the speed of sound is about 1125 fps, so the sound reaches them long before an arrow does!