What are the causes of Crohn's disease and ulcerative colitis?

Genes+environment . A combination of genetic predisposion (for example crohn's is more common in ashkenazi jews), life events (eg appendectomy increases one and decreases the other), and environmental influences (eg smoking increases crohn's and decreases severity of uc...). Bet you didn't expect the latter! as knowledge increases more precise factors are identified.
Immunologic. While no one knows the exact cause of inflammatory bowel disease, there is an alteration in the immune system, causing our own antibodies to target our intestinal tract. The extent of inflammation is variable, and there may be significant repercussions in both of these diseases.