Think it was due to being molested multiple times by two different men. I was young and I think it made me want it at as I got older.
For me it was that I was always looking for enhanced pleasure and orgasms which led me to butt play and tasting my own cum...just kinda went from there I guess..
For me, it was all about exploration, realizing that I could not just stroke my cock, but play with my balls or even play with my ass, and started to wonder what it would be like to suck my own cock (which I could never do by the way, I'm NOT one of those people, not flexible enough), and then started thinking how I would totally like to try out sucking a cock, and having somebody give me anal pleasure.