IT´S A BUG.
From the IGN interview with Rockstar. Skip to 2:25
Originally Posted by Rockstar
"Its something as an art team we focus on a lot break up the hard edges, break up the straight lines and you get something much more real. We then updated all our bushes, plants and trees with much, much more detailed equivalents and then worked on a much more advanced wind system so the branches and leaves blow more realistically. We worked on a layered approach to the grass, starting with hand placed weeds along the bottom of fences and walls, complete with little bits of litter trapped in between, then we layered a base level of grasses of various types over the world."