Yes it is a coding thing. It is in some of the maps you can download. I looked everywhere though and couldn't find the actual file for it. But I downloaded a few maps and now almost all of my trees sway. xD
Sorry I couldn't have been of more help.
i remember on my old computer i downloaded some type of moving meshes!
i did a quick search for them, here u go!
http://giddyfox.deviantart.com/art/Grass-Pack-for-FeralHeart-417145702
i think the grass moves in this one, try it out