It happens because she has no reason to feel anxiety next to a guy she doesn’t like or value, if she’s not feeling her most confident that day and your aura or look exceeds what she thinks she’s capable of matching at that time.
She’d rather not put herself in the position to feel feelings of insecurity for the duration of that ride.
In her mind at some point he’ll have to talk to her to get by, or he may talk to her and she’s already decided in her head that something about her is too wrong and will give him the “ick” and women hate to feel like they give a desirable man the “ick”
It’s just defense mechanism but it won’t make sense to men, even at their worst men would sit next to the attractive woman before the unattractive women..cause ,..men, we got a shot or no shot at all and we accept it
Focus on other things…