I have wondered about this for a long time myself. My first idea was that their visual systems were not equipped to process moving scenes, so they moved their heads backward at the same (relative) speed as their bodies moved forward, and then snapped forward, giving themselves a series of still views. I have never been able to observe them carefully enough to check whether their heads are in fact stationary during the rearward part of the cycle.

Another whimsical thought was that maybe their pecking muscles need the exercise to keep in shape.