Mine has only covered frags when it is
1.Feels as some is a threat to it
2.Or if something is covering its tunnel/entraces
I know whenever I add new frags to the tank,around the pistol's hole, he will pop his claws for a a while,I guess they feel like they are a threat,then I wake up the next morning and they are covered in sand.
I think it is a neat pair to own,because they usually stay side by side which is neat to see with a little fish and a little shrimp.