They're not mining rocks - that is supposed to be grass :} And that problem has been occuring for as long as I can remember, too, with sometimes what seems like random bursts of "Hey it works!"
this happens to the grass often, sometimes the big boulders, and the cliff behind harnquists blacksmith shack, a simple reload of the client fixes it.and hey, where you took that screenie... be grateful you didnt experience the technicolour doom tunnel