It's not like we jump up and yell our intent at other times.
Like, when guests arrive at a planned dinner, we don't yell "eating"
When we're getting ready to go for a walk, we don't jump and yell "exercise".
However, it is still a wise idea to yell surprise when you're doing something unexpected, like butt sex. Or any surprise sex at all. I often yell out surprise five or six times during sex, just because I want to make sure my partner knows it was an accident. She orgasms, "surprise!".