My maroons do this all the time. They seem to be diligent housekeepers, making sure that the anemone is clean and that the substrate is kept away from it, even to the point of having 4" tall mounts of sand around the anemone. I have 2 theories on this, first being that the more they dig out, the more room they have to retreat under their host for protection. Secondly, I think they may be digging for a suitable spot of deposit eggs that is under the protective umbrella of their host. Every few days I push the sand back to an even level, and they start the cycle anew.