Some causes are :
change in diet,Dairy or other food intolerance ,Ingestion of spoiled food
Hairballs, Allergic reaction, Bacterial or viral infection, Internal parasites, such as roundworms, coccidia and Giardia, Inflammatory bowel disease, Kidney or liver disease, Cancer or other tumors of the digestive tract, Certain medications, Hyperthyroidism, Colitis.
Some ways to stop/help it:
canned pure pumpkin (not pumpkin pie filling) is supposed to be good, or pure sweet potatoes, half teaspoon of Metamucil, and give them more water.