Mites can definitely be a problem and drive them from their coop. But have you checked the birds to make sure that is the issue? Also diatomaceous earth has very little effect on parasites and can be very harmful if inhaled (to both you and your chickens). [Typed some of this before some other people replied and just now had a chance to post it.]
Here is a link to a thread that may help…
Two more issues might be light and ventilation. You said you blocked off light from their coop, so they still have enough natural light inside to be able to get in and get “ready for bed” before sunset? And is there sufficient ventilation? (I think the recommendation is at least one square foot of ventilation per bird. I can’t remember who it is that usually posts all the good specs and info on that. What kind of bedding are you using?
And what age are the birds? Rising hormone levels in “teenage” birds can also contribute to a lot of things.
Tagging a few more experts for more advice.
@azygous @Mrs. K @3KillerBs @Wyorp Rock @aart