You are witnessing what junglefowl would be doing in the wild.
After the mother is done with showing the chicks what she thinks they need to learn, she will chase them away. After some convincing, they will leave the group they grew up in, to form a group, or join other groups of their own. In our domestic setting, much of this still applies.
Our broodies still chase away/wean their offspring after they decide that they have learnt what is needed. Sometimes, however, the juveniles physically can't form their own group, possibly because they are kept in one coop all day without free ranging, or possibly because there is no second coop, so they can't have some distance from the senior birds.
In most cases, the birds work it out in the end, especially when the younger pullets reach laying age. Still, sub-groups can be detectable.
Each hen has her own way of weaning her chicks. Some are very mild with their weaning, others chase them away with pecks and a few kicks, but some just can't stand living in the same pen with the offspring they raised, no matter what.
Having a large coop usually helps; the youngsters roost on one side, and the adults on the other. When the youngsters grow older, they might start joining the adults, as mentioned before.
Your coop is just too small. Mum can't have her space respected in such a small space, unfortunately, and it seems like the chicks were persistent enough to kick her out. Having a second coop seems like the only scenario for now, at least until a couple of the pullets start laying