How old is that rooster? Is he really just an immature cockerel that has not yet taken over the job of flock master? In a flock that does not have a dominant mature rooster it’s fairly normal for one hen to take on the position of flock master.
The mating act with chickens is not always about sex. It’s often about dominance. The one on bottom is accepting the dominance of the one on top, either willingly or by force. By going through the mating ritual, a chicken is demonstrating their dominance.
I had a flock that consisted of some older hens, some pullets, and a cockerel. One hen was clearly dominant. She would occasionally but not often mate with another hen or usually a pullet that was old enough to lay. If she saw the cockerel trying to mate another hen or pullet, even if that other one was willing, she would knock him off to show that she was dominant and the only one allowed to go through that ritual.
Eventually he did mature enough to take over the flock master duties. That top hen resisted and there was a bit of fighting and chasing, but eventually she accepted his dominance and they became best buddies.
Bottom line. While a hen mating another is not typical, it’s not that unusual if no dominant male is around.