What is the pathological process that results in fistula development?

Trauma/inflammation. Any source of trauma or inflammation can result in the development of a fistula. Appendicitis, crohn's disease, gun shots, stab wounds, surgical complications, diverticulitis-- these are all potential causes. Basically, when trauma or chronic inflammation causes the barrier between 2 spaces within the body to break down, the resulting connection is called a fistula.
Many types. There are many types of fistulas. Need to be more specific.