It's what they do (same as many other large cichlid species). No matter how big the rocks are, they'll move them to suit their territorial design. I've had large fish move rocks heavier than them by undermining smaller stones from under the larger boulders. Don't try to outsmart them by putting in monstrous boulders. Aside from robbing the fish of too much space, you'll also run the risk of blowing out the bottom of your tank.