Actually, this has got me thinking - why would you want to build amound out of turf, or rip up your precious grazing land to do it?
But if you wanted a mound, and happened to have an intention to turn some land over to crops once you'd got the hay off it, you'd be left with all that free turf, and you've got to clear it off and put it somewhere so....

Or a compost heap?! That'd work, and would explain the successive layering.