Just been reading Burl's Rites of the Gods, and he points out, that in some sites soil and debris from their living site is also found under mounds. It's a sort of a fertility rite, bringing everything back for renewal. I expect the layering of mounds may point to a more methodical approach of building a cairn but the idea that the earth needs to be magically renewed to grow crops would be bound up with the burial of the dead. He's quite strong on child sacrifice as well during early/ middle bronze age, seemingly as a foundation offering when some barrows/henges were built.