My guess would be she's lowest in the pecking order. Least likely to tell them off and fend off their advances. Then when one corners her the other wants to join in.
The hope would be he would be gone long enough for the hens to forget him and he'd have to work to be flock leader. And hopefully forget about your Orpington. I can't guarantee it would work, it's the only option I can think of.
Did the Orpington hatch eggs from your flock? If you want a...
Culling the more aggressive rooster sounds like a good plan. Are any of the cockerels of interest to you? You could process both older males and introduce a young cockerel. The hens could teach him better manners.