Unless you actually see mites, I would guess that Shezadandy has come up with the most likely probable cause of the change in roosting behavior - it's just cooler outside.
X2
Pictures of your coop/run may be helpful as well. How much ventilation/windows do you have?
Is there a way to place a fan that blows air through the coop for better air circulation?
I have one in my coop window,and it is usually 5-7degrees cooler than the outside temps during the day and anywhere from 10-12 cooler at night. I do set it to go off if it goes below 55. But we do have large temp variations, so it can be mid 90s during the day and drop to low 60s even 50s overnight.