All my spawning pairs do this, they clear all the sand away from the flowerpot and pile it up against the front glass. In every tank there is a bare bottom area in front of the nest. I'm sure there are a number of explanations for this but I like to think that having a clean surface rather than a sand bed makes it easier to spot predators approaching ( snails, brisstleworms, etc). I constantly see females picking up snails and carrying them to the farthest corner of the tank where they drop them.