I'd say one factor in the adoption of a fetish is incorporating it into some sort of humiliation or into a taboo act. Washing your hands isn't really shameful in any way other than maybe that you got your hands dirty. Fucking bugs is taboo and might be humiliating. Vore is definitely humiliating in the power play sense and getting eaten is technically a taboo. A lot of transformation stuff (socks, clown) is humiliating (e.g., "silly", "dumb" for clowns).
So it's what happens when someone connects sexual connotations to a nonsexual taboo or humiliating act. Then you can progress from the sexual aspect you see in the nonsexual act to sexualizing the nonsexual act itself.