Have you ever actually seen your own ass?
I mean, in general, if you see someone's ass on a regular basis, it's most likely not your own. And you probably get used to it. But seeing your own ass is a rare enough event to be somewhat shocking.
It's kind of like going to a hotel for the first time that has a full-length mirror on the back of the door so that when you step out of the shower and see your entire body naked for the first time. Except that when you do that, you still have the frame of reference of your face.
With your ass, you have no frame of reference. It's just there, like some alien planetosphere that sways with you. It's like some odd naked person is doing an acting exercise with you where they have to mirror your every move.