Many cats like to eat grass. In many cases it has a mild laxative effect which somehow they know will help flush out there system. I would believe if this is the case, it would help with eliminating fur balls from grooming and ingesting their fur. Sometimes it may also be a strange fascination of movement that they try and catch long grass when it is waving and try and stop and eat it, much like when they are playing with a toy or chase a moving string or curtain thread etc.
Generally a dog and cat may ingest more grass or plant material when there is some gastrointestinal discomfort or upset. It is normal though in many cases.
Just to be on the safe side, I would take a sample of poop to your veterinarian to ensure that there are no intestinal parasites and if that is clear I would ensure that they get a dose of laxative paste 2 x weekly to ensure no hairball irritation or constipation from grooming.
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street Units 40-42
Tel. (905) 771-9855