The sand is wind blown, as it flows over the surface irregularities in the surface, or eddies in the wind slow it down in places and the sand is deposited on to the ground, this causes further flow disturbance and the sand pile expands upwards (in height).
The mini dune will have sloping sides, the side into the wind being longer and less steep than the leward side, and there is a limit for the steepness of the slope which is maintainable with dry sand(not sure what it is but look it up).
If the slope is greater than the limit, the sand will roll down hill spreading the dune along the ground (in the direction the wind is going).
This fills in the gap in front of the dune and raises the surface level, allowing more sand to build up on top, which then begins to move forward in a repeating cycle.