We've had a problem similar to that GSH. The hens started attacking the cockerel's favourite whilst he was treading her. She took a hell of a beating and was pulled out permanently. Same happened to his next favourite, so we pulled all his hens out permanently. They are all strong and healthy TNN's.
On reflection it may have been a run size issue triggering the behaviour. TNN's are not large birds, but 4 hens and a cockerel in 5 square metres (with a 2 metre perch) is far too small for such an active breed.