The wielding animation of the undead chicken was updated in June 2009 to be a smoother version of the "idle" animation before the huge graphical overhaul. However, for a long time after the release of Runescape HD, the graphical animations of a player while wielding an undead chicken were not updated along with the rest of the wieldable weapons.
Both the old and new animations for the Undead chicken made players turn their head to the right every so often.
It is thought that Jagex would want to keep the "head turned to the right" animation for historical purposes, as they revamped the old animation to be smoother rather than replace it.
As of the Evolution of Combat, the Undead chicken is one of several weapons which will cause the player character to have the old idle and running animations when equipped. The idle stance for the Undead chicken is not unique, however, as the character no longer turns their head to the right occasionally.