That has been happening to me quite often, but not consistently, and it only started recently. I have especially noticed it when I was painting whole lots with sand. (Making beaches, obviously. I really should consider installing Bon Voyage.) It must have something to do with switching to a cursor that matches the terrain brush.
Thought: Then next time it happens, try cycling the Caps Lock key. In Photoshop that's the key that controls whether or not Photoshop shows the brush shape as the cursor.
