Suddenly changed roosting area

I have 4 distinct groupings of pet chicks. The older two are grown gray bantam cochin, roo & hen & they have their own coop. The other are grown bantams (not cochin) roo & 2 hens, they have their own coop. Then grown white bantam cochin roo & 5 hens. And finally 4 young bantam (not cochin) 3 cockrells & 1 hen that came from the other not cochin group. So here's the thing. All the coops are located in a large fenced in, protected area so they can be out during the day. I alternate the 1st bantam cochin & the 2nd not cochin roo to be turned out as they fight constantly (learned this after the first & only trial). All the others get along fine.

However, at night, they are all closed up in the coops for safety. Here's the problem. They were naturally roosting in their own coops until recently when all 6 white ones decided to stop getting in their coop (which don't have a roost as they all stayed in the corner together), and started roosting in the coop with the other growns. The youngest ones used to do this (as they were actually raised in their anyway & only moved to their own coop when they got older but still weren't say to be let out with all the growns), & that wasnt' a prob. Everybody would at least be in a pen, I would go down after nightfall & just close the coop doors & lock them up. Now, the whites won't let the youngest ones into the coop they have taken over, & they roost on top of the coop. So, I have to go every night & put the youngest ones in the coop that was supposed to be the white ones anyway, & they don't really like it (but it's less time consuming to move the 4 little ones than the 6 white ones!).

So my question, after this lengthy explanation (sorry), is: why did they do this all of a sudden, & how can I get them to go back to the simplier way of life!! Thanks!