That question is posed a lot so it does not offend.
I think it is mainly the taboo of it all. Our whole lives, we are told flatulence is gross, disgusting, and people who do so should be embarrassed. We go out of our way to cover it up and act like it doesn't exist. We blame it on others when it is brought up in a room of various people. All this applies to everyone, especially women. Women get it the worst. They are told they absolutely CANNOT fart in front of anyone for any reason. Society tries to make women seem like they are inhuman, with no bodily function whatsoever.
But why? Why should they be embarrassed? It is perfectly natural. And part of it's taboo is why is turns some of us on. That and when a person farts, they are instantly relieved. If you look at their face, it brings them joy to let it out. Also,the sound of it is pleasing. Plus, farting is very personal. It is a person showing you that they are comfortable around you and are allowing you to see a more natural side to them. As for the smell, it is rarely pleasant. But the fact that it's so dirty and naughty, turns people on despite the fact that it usually smells bad.
Couple all of that with the taboo of it as well as the fact that it relieves a person and makes them feel good and you have a fetish.