In my experience, I was told my RV fistula was caused by an abscess/infection in a rectal gland, and that it had nothing to do with my pouch. My pouch has always functioned beautifully (for 10 years), no issues, not even pouchitis, until this. I have to admit, I was very worried it was Crohn's, as the infection had me in the bathroom so much and I felt so lousy (just like with UC). As soon as the infection cleared, I was much better. I have since had my fistula repaired, and so far, so good.
Fistula's are most common in people who have IBDs, but the other most common cause is childbirth. It is not unheard of in the "general" population.
As for the seton, akteacher, it is considered a drain, and keeps the track open to prevent it from becoming infected. In terms of a rectal/vaginal fistula, think of it in terms of a rubber band. It loops through the fistula track (or hole), and then snakes down the vagina, and back up through the rectal side, and tied off. Some people have no issues with them and don't even feel it. I, personally was very uncomfortable and irritated by it. . .
Best of luck ladies.