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1st, they are omnivores and feed on algae and other micro stuff in the environment, so often they are cleaning algae off the gravel pieces as they move them.
2nd, most like a clear, flat spot for spawning, so they could be preparing a spawning bed.
Because they're cichlids, and one thing just about all SA or CA cichlids have in common is their annoying habit of rearranging the tank to their own liking. Just be glad they don't have opposable thumbs...