A very old mystery, and perhaps we have to accept that some mysteries will never be solved, but at least that gives us freedom to speculate.
It's an odds on bet some twerp will come along with the totally ridiculous idea that the remains were buried there out of some religious ritual or rite.
Some twerp always has to add the religiousness to how our most ancient ancestors would have lived but I sincerely doubt that ancient Hominids would ever bothered to cast a thought towards there being some Great, Mysterious Entity/Entities since their only drive would have been to eat, breed and survive thus ensuring that the future generations would be born.
Yes the ritual think always seems to be a default possition for some archeologists, sometimes to a laughable degree.