Hi there!
This "phenomenon" happens because of erosion caused by wind and deposition. Wind picks up grains of sand and deposits it when the sand encounters an obstacle (in this case, a sand dune or the like). Because the sand is not held in place like soil, the dunes are constantly changing, allowing for varied shapes and sizes of the dunes every year Eli and his family visit.
I hope that helps you out! Have a great day! :)