Spend 10-15 minutes watching your chicks' behavior very carefully. My guess is: they're pecking each other. Do you see some running around with feathers in their mouths, being chased by others, and then eating the feathers? Can you catch one of them in the act of grabbing a feather out of another chick's back?
I have this problem with mine (who are 5 weeks). A few of my girls look exactly like you've described yours. I've separated out 2 of the most aggressive peckers (mean little RIR girls who just like to pick on my sweet, easygoing EEs.) This is important because pecking can be a learned behavior -- so if you can isolate the ones who started doing it, then it might not spread to the rest of your flock.
I'm also going to be supplementing their feed with some protein in the form of sunflower seeds and yogurt. (Sometimes a lack of protein results in pecking/feather-eating behavior.) You can also put "saddles" on them so they won't be pecked; I haven't tried this yet. Also, make sure they have enough space and entertainment. Sometimes they'll peck at each other if bored. For mine, treats like zucchini and sweet corn seem to distract them from feather-eating.
(I've been researching this topic a lot, so most of this comes from other BYC threads and google searches, just so you know!)
