Amato's right in that there is no known consistent cause or triggers of colitis.
Elaine Gottschall, author of Breaking the Vicious Cycle, claims that avoiding all simple carbohydrates for an extended period of time can heal colitis in some people. She outlines a very strict diet that I followed for a while. The book said not to bother continuing if there was no improvement after a month, so I didn't continue after that long.
You're welcome to try the diet. Check out her website, and pick up the book.