I have noticed this about chickens. We mainly have kept Rhode Island Red s for egg laying (solid, sturdy and hearty prolific egg layers). If we introduce another hen into the flock that looks vastly different from them, they do not allow them into the flock. We have some small bantys our son and his girlfriend raised just because and they, of course, come out to the field to be with the other chickens....we have one white Japanese bantam, the kind with the black tail, and they have never let her into the flock, she has to run in the middle of them, grab something and run out with it. Same with the other lttle ones.
This seems to go with all the lighter colored hens that are even regular sized. My guess on this is a "natural selection" thing. They only want other birds they see as same as them or close to the same, in the flock to keep those genetics going. Thus, when you introduce a chicken vastly differing in size or color, they will not let it in and it has to live on the outskirts of that flock to some degree.
Then there are the times you have the same breed being introduced and they are beyond the normal awful for a short time to the new chickens. This could be that there is either not enough physical space, or, not enough feeder space. If you have one feeder and twenty chickens, they will see it s this is all they have and they need to protect it from others, not knowing or understanding you will refill it as much as needed. They will see it as a limited resource for their flock that needs to be protected. So more space or add more feeders and waterers to match their needs, see how this works.