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I'd have to know a bit more to come up with some ideas - do you have hens and roos? Is it just the hens that are losing their feathers? If you just have hens - is there one that isn't losing her feathers and all the others are?
My first thoughts are from mating (if you have a roo and it is the hens that are missing the feathers), disharmony in the flock (have a short one running around plucking feathers?) and possibly - if your nests are a bit tight for your birds -- they may be damaging their feathers when they get in and lay.. I know most of the nests that are sold pre assembled are way too small for my birds.. we made our own..but if yours are short front-to-back, then they may simply be pressing p against the back when they lay, bending and then breaking the feather shaft behind the tail..
Sounds like something that would be good to post on the managing you flock topic - and if you have a pic, that would be helpful.
Best wishes on figuring this one out!!!!