Frustration. There is an outside intruder that the cock wants to chase away from his territory and he can't get to it so he gets frustrated and starts beating his hens. Which is ironic because he was trying to protect his breeding stock.
Not long ago I posted a thread about what is in your medicine chest, well, we had an emergency yesterday and we were much relieved to have the supplies we needed to help out our hen. Lil Bit, a high Spalding Dark Pied hen we have from Bob's Green Peafowl was attacked by her White Spalding cock...