There are different possible reasons this could be happening. Each chicken is an individual and each flock has its own dynamics. I don’t manage mine the way you manage yours either, but we are also all unique. I can’t argue with anything Barbara said.
With them all being one year old I’ll offer a different possible reason. How long has this been going on? Some cockerels don’t reach sufficient maturity to be a real flock master to all hens until they are a year old or even more, though most do. I’ve had a situation where I had a hen as flock master while a cockerel was growing up with the flock. As the cockerel matured the hens one by one accepted his dominance, except for the dominant hen. She just would not give up her dominant position. The cockerel did not challenge her at all until he was pretty mature, somewhere around nine months in my case.
Then he decided he was going to take over but that hen was having none of it. For two solid days that cockerel went out of his way to be brutal to that hen. He would chase her and peck her, normally around the head. He was using brute force to get her to accept his dominance. After two days of this she gave up and accepted him as the dominant chicken in the flock. They became best buddies.
Once pecking order and flock dominance issues are settled a flock is normally pretty peaceful, but getting to that peaceful stage can sometime be pretty brutal.
I going thru that exact thing right now. I have 8 hens and no rooster I got a rooster that's about 6 months old. He is bigger than my hens. But the dominant hen is the only one that messes with him. She will chance him around the pen and has to jump up in the air to peck him in the head. I got some of those peepers I'm going to put on her if that doesn't work I'll take her out till he gets a little bigger.