I've seen that too... It could very well be the grass following the grid lines. Have you tried adjusting the dispersal value up a tiny bit? I think that you happened to have hit a place where the grass so happens to fall on the grid. Remember the grass clumps actually grow out of each grid point on the mesh.
I will say that it's not that bothersome to me, however, because it looks like tractor wheel marks or old lawnmower marks in the grass.