If the hen is older or was previously dominant over him, he has finally felt it was his time to be a Rooster and be dominant over hens and breed them. How they go about it varies, some simply challenge hens and fight it out if the hens don't back down like yours.
What you need to watch out for a situation like yours is make sure the other isn't actually another male and with either male or female, watch how he deals with the loser after winning. If he continues to beat on after the loser has given up and is running around or trying to hide that is bad and you need to separate if they are confined to a cage or a small coop.