As someone who formally studies psychology, the main point I should get across is nobody 100% knows for certain what causes fetishes of any kind. Even when you ask people with these fetishes why and have long personal conversations with them, they really don't know themselves. However, there are a lot of theories and general correlations that have been noted.
One of the more universal theories is called Repetition Compulsion, which is a concept where when you're a little kid and something scares you or makes you feel out of control (real events or movies), you become fascinated with it when you're older. One of the most well known is sadomasochism: Most (but not all) people who are into it were physically struck as children (spanked, even if it was not abusive). The crazy thing about this concept is people have no idea they do it.
Another concept is how fetishes are a way to hide from emotional intimacy. If what you're into is really weird, the sex act becomes almost mechanical rather than a expression of love and emotional closeness.
And there's a few where it's not a true fetish, but rather the expression of brain damage or mental illness. For example, the really crazy ones involving bodily fluids or unwilling partners (like children, the dead or animals).