Cats have very sensitive digestive systems. It doesn't take much to upset their stomachs, unfortunately. Common causes are: hair balls, food changes, eating plants, eating human food (especially dairy products if your cat's not used to them), and eating too quickly. There are, however, illnesses and diseases that cause cats to vomit: chronic renal failure, feline infectious peritonitis, etc.. If you find your cat is vomiting frequently and losing weight, a check up with your veterinarian is in order.