When my bp was rubbing, that's the spot where he kept injuring himself. I wouldn't worry about it unless it keeps happening. Any snake will try to escape from it's enclosure. I don't care how nice it is. If she keeps doing it, then you'll have to consider making changes to her set up.
My Fonzie injured himself repeatedly. He would rub until he got a nice lump. Then, he'd stop until the swelling went down, and then, he'd go right back to rubbing again. He was in a tub, so I build him a much larger vivarium. He never stopped trying to escape, but he did stop injuring himself.