I have four RIR that are 10 months old. they have been doing very well in their pen. They were all from the same raised flock. I have no rooster. Eggs have been steady (3-4 per day). I found one of my hens dead, with noticeable injuries to the back. The pen is predator proof. I believe she was killed by another one of the hens (or maybe the three remaining). What would make the another hen do this? The only thing I've changed recently is placing a sheet of wood as a wind breaker on one side of the pen.