It's possible you're cat isn't getting the vitamins and minerals it needs in it's diet? What type of food are you feeding? Be sure it's a good quality human grade food, with a proper amount of wet food.
Otherwise, cats will eat grass if their suffering from an upset stomach. It's a way for them to induce vomiting.
Also, they seem to like it. :-)
There is stuff called oat grass, ( i call it cat grass). It is safe for them to eat. I started to grow it so that my cats would stop eating my plants. I just grow it in a small rubber bin and put it by a window.
I also have noticed if i don't have it out for a while and then put it out when they first start eating it they do throw up but after a while they stop.
Hope that helps.