I can only speak for myself but,
Its literally the one time I have to give up all control to someone else.
To lay there, naked with legs spread, waiting for a stranger to do things to my most intimate areas while another stranger watches and/or assists is almost too much to handle.
Knowing that anything that is going to happen is out of my control, no matter if its a fast procedure or a long one, Not knowing if its going to hurt when they do it or not. Or knowing that is going to hurt and having to suck it up and take it.
After being taught for years and years to never let anyone "touch" you, then willingly exposing yourself for someone to poke around and insert things...the very contradiction of it is a turn on
I myself am a huge baby when it comes to injections or blood being taken in real life, needles just freak me out, I turn ghostly white and just about pass out. Yet, its a major fetish for me and is a big part of our play.
In that aspect its fear that charges the fetish.
signed..
"the wife"