Carol;

Four things can uproot moss.

(1)Birds during the nesting season when birds need nesting material and (2) the rest of the year when birds are hunting for invertebrates that can live in the moss.

(2) High winds and or (3) driving rain when moss is well exposed to these elements.

Many soils are basically small fine particles. Moss has rhiziods that do not go very deep into the soil, thus moss is anchored to only a limited degree. Sometimes when moss is exposed to the elements and moss is not a large carpet but only scattered bits and pieces then driving rain and high winds can uproot the moss and blow away.

Solution: Cover your moss with very fine black mesh that can be bought at Wally World or any other store that has a fabric department. This inexpensive mesh is very hard to see when you are observing your moss garden from eye level. Even when you are hands and knees it is not a distraction.

When your moss has formed an extensive carpet you can remove the mesh after the nesting season is well underway.
Rick