Welcome to the forum, glad you joined!
Some good questions so far from others. I'll also ask their age and sex. And where were they sleeping inside the coop before they moved to the top of the coop?
Chickens tend to roost on the highest thing available. It could be the things others have mentioned, it could be that the top of the coop is higher than the roost inside, especially if they have just started to roost.
It is also possible one of your chickens is a bully and beats the others up if they try to roost with it, especially if your roost area is tight.
It's hard to tell what is going on across the internet, sometime seven being there doesn't help a lot. The more you can tell us about your set-up and situation the more likely we can help.