Family History
Apparently up to 20% of people with Ulcerative Colitis will have a family member who also has the disease. This could be due to genetic factors, where by there is an ulcerative colitis gene which is activated in some family members, or perhaps caused by environmental factors, such as bad diet and lifestyle within a family which leads to ulcerative colitis manifesting.

Geography
Incidence of Colitis is greatest the further you get away from the equator, which suggests a link between sunlight exposure and colitis, Sunlight produces Vitamin D which a lack of has been linked to Autoimmune conditions like Ulcerative Colitis. It is also much more prevalent in Europe and North America than in other parts of the world, which would fit with a link to sunlight exposure.

Autoimmunity
Ulcerative Colitis is commonly thought to be an autoimmune condition, where the body’s immune system is reacting against itself. In the case of Colitis the immune system reacts in the wrong way to bacteria in the digestive tract, and that immune response leads to inflammation of the colon and ulceration. Autoimmunity has been suggested to be triggered by a ‘leaky gut’ whereby food proteins cross the gut barrier intact causing the immune system to attack them.

Environmental Factors
Ulcerative Colitis is statistically a disease of non smokers. It is more common in non-smokers and ex-smokers, in fact nicotine patches have been tested with some success as a treatment for Colitis. Working & Living environment has also been suggested to play a role in development of Ulcerative Colitis – people living in cities and those who work in office environments seem to have a greater incidence of the disease than people who live in the country and or work outside. This is another indication that lack of sunlight exposure may play a role.

Bacterial Infection or Virus
It is possible that a viral or bacterial infection could be responsible for triggering an immune response, which continues after the infection has passed, possibly due to a genetic predisposition whereby the gene is triggered by the infection but fails to switch off after it has passed. Or perhaps a particularly stubborn pathogen that is never quite destroyed by the immune system.

Bad Diet
It is quite possible that a diet excessively high in sugar, alcohol and grain based carbohydrates, can lead to a disbiosis of intestinal bacteria, meaning an imbalance between good friendly bacteria and bad disease causing bacteria. This may initially show itself as foul smelling wind and general bloating and discomfort, ultimately leading to Colitis if things get really out of control. A bad diet high in sugars & carbs -- typical in most western countries -- is also likely to set the stage for a leaky gut which is another possible cause of colitis.