Question

what causes the sand dunes to shift from one place to another

Answer 1

Although no more than a pile of wind-blown sand, dunes take on a remarkable range of sizes and shapes, depending on the amount of sand available, the size of the sand, and the prevailing wind directions. 
Over time dunes can grow, shrink, or move in the direction of prevailing winds. 
Dunes can be created and destroyed by either nature or humans. 
Dunes can roll over trees and buildings, or be washed away by storms.