Cat food is very high in animal protein so it doesn't take much. Lots of folks use it as a molting supplement.
I have only seen this on a single chicken before and it wasn't mine so my knowledge is incomplete. However, what I do know is that it was the lowest one in the order being plucked and there were multiple hens pulling feathers from her lower back when she'd submit. The problem was solved by reducing the low protein "treats" that were being too liberally offered and giving them a little cat food for a treat instead. This is why cat food was the first thing that came to mind.