Subscribe to our Digestive Health e-newsletter to stay up to date on digestive health topics. The exact cause of ulcerative colitis remains unknown. Previously, diet and stress were suspected, but now doctors know that these factors may aggravate but don't cause ulcerative colitis.One possible cause is an immune system malfunction.reviously, diet and stress were suspected, but now doctors know that these factors may aggravate but don't cause ulcerative colitis.