If there are no little bugs running around, which can also cause feather loss, she looks like she is molting to me as well.
The only other possible option, if that is her breast (hard to tell in photo), she might be plucking her breast in contemplation of brooding. Has she been hanging in the nest box a lot? Cranky? Puffing up when others come near her? My broody hens will pluck the feathers off their breast when they begin to brood. often leaving loose feathers. RIR's are not commonly a broody type breed, but it can happen even in commercial production lines.