There is an interesting discussion of decapitation traditions in
Adrian Bailey, The Caves of the Sun: The Origin of Mythology, especially Chapter 12 and following. The gist of his argument is that in many ancient cultures the head was symbolically associated with the Sun and crop fertility, and decapitation after burial could therefore symbolize the "setting" of the soul at death as well as the decline of the Sun in winter, with the anticipation of a "rising" in the afterlife and likewise of continued agricultural fertility the next year. Horses (linked to traditions about chariots pulling the Sun) also were often buried decapitated among Indo-European peoples.
Daithi O'Hogan, The Sacred Isle: Belief and Religion in Pre-Christian Ireland, p. 47 mentions an apparently related tradition practiced in ancient Ireland involving the burial of an animal skull--usually a horse--beneath the foundation of a new home.