Ten birds in 25 sq. ft. is way too tight! Reevaluate your feeding program also, and either greatly enlarge your coop, or get rid of your aggressive birds. I have about five sq. ft. per bird in my coop, and that includes many bantams. Mary
Ditto Dos^^^I'm surprised you've not had issues sooner. Minimum space is generally considered 4 square foot per bird in the coop and 10 square feet per bird in the run. These rules are not hard and fast, but your birds are absolutely telling you they're too crowded and need more space. You might be thinking that's not the issue cause they've been fine for 2 years, but IMO that was pure luck. Agree with Mary you need to either enlarge your space or cut your flock by about half.