Brahma rooster is bullying one hen in particular for some reason

[Sire12]

I have a flock of 10 mixed breeds he is the only roo, he's 14 months old and became head of the flock about 5 months ago, since then he's been picking on the ex head of the flock my red star every time I see him do it I discipline him like I did when he pecked me during his coming of age phase, now he's as gentle as a puppy with me and let's me pick him up and pet him he's very good with the other girls too never pecks to assert dominance or anything except for this one red star he absolutely hates her, I don't think I've ever seen him tread her either she will squat for him but he just pecks at her and chases her away especially during feeding time, does he just hate her since she was the ex head of the pecking order and is possibly getting even with her for pecking at him when he was growing up? Or possibly since she's the only red he just doesn't like red hens or something? Is there anything I can try to stop this behaviour, attached pic is him with my jack russel

Thanks

 

[aart]

I've had this happen, never really did figure out why.
The bird getting picked on was not laying and molting.
Luckily I have enough space for her to get away,
she started hanging out with the younger birds.
Eventually he got over it and all was well again.