Weird aggression between calls, what are they doing?

My DS has a two-year old pair of calls in a "tractor" type moveable pen that sits on the ground. Last week I finally started allowing my trio of Orpington/runner crosses to free range. Of course they're often going over to the call pen to visit, but the calls always get extremely agitated when the big ducks come over. They can't get to each other through the 1/2" hardware cloth, so I'm not worried about that. It's the female call's behavior that seems odd. When the big ducks come over she suddenly starts picking on her mate, pulling his feathers and acting aggressive toward him. He's not usually henpecked -- I just witnessed them mating and he was definitely in charge -- but when the big ducks come over she turns on him. What's going on? Should I be concerned? As far as I can tell she hasn't hurt him, but there are definitely more white feathers in the pen than there were before the big ducks starting coming to visit. I don't have an easy way to prevent the big ducks from coming over (I'm committed to letting them range), but if there's a chance she might actually injure the drake I guess I'd need to figure out something. We're new to ducks, so I'd appreciate any advice.