I was told by my geography teacher 40 yrs ago it was the small soil particles slipping below the larger ones. movements of the 'Earth's crust causing the slight shakes. And our cookery teacher said that if you shake the bowl when rubbing fat into flower the bigger lumps come to the surface so you can see if it needs more rubbing in.
sort of makes sense.

This explains it perfectly
https://io9.gizmodo.com/the-brazil-nut-effect-why-the-biggest-nuts-rise-to-the-5786442

I'm sure you've heard of plant heaving - when plants are literally cast out of the ground due to the freeze/thaw process. The same happens to stones and rocks. The ground is not inert.

You'd surely find this a fascinating read:

The Sailing Stones of Death Valley
https://www.nationalparks.org/connect/blog/sailing-stones-death-valley

Thanks to you all. I’ll follow those links as it does sound interesting.