Eating grass can also be a sign of a nutrient deficiency. When I first brought home Jaina, she would almost constantly eat grass. I had a fecal done as required by her daycare, and it turns out that she had a nasty hookworm infection (which, in case you're wondering, a hookworm infection can cause lowered iron). She never threw it up, but her poops were half grass and half poop much of the time.
Now that the infection has cleared up completely, she never eats it anymore. And her poops are now just poop