^ or too much green. The chicken droppings provides the nitrogen rich source that makes the deep litter compost work. The brown, dried leafs provides the counter balance that keeps odor down and keeps the system working as intended.
You can throw kitchen scraps, etc onto a deep litter system and not whack it out of balance, assuming its sufficient size and there aren't too many chickens for its area - but if you start dumping bags full of grass clippings approaching a 50/50 green/brown ratio like a hot compost pile, you have the potential for LOTS of problems. Typically mildews and molds, mushrooms and the like. Sadly, our chickens have surprisingly delicate respiratory systems.