Why do people gag during regular blowjobs?

I don't mean deep throating because that makes sense, but when girls are giving regular bjs why do they gag? Is it disgust? Taking more than you thought you could? If you just have the head in your mouth why would that make you gag? I thought gag reflexes were only stimulated at the back of the mouth. I'm trying to avoid doing anything that causes gagging during a bj 😅