This is very weird. Never seen that in my childhood and I've seen a lot of chickens, my grandmas used to have them in the yard, and I would feed them and clean around after them sometimes.
However recently I have seen a video on YouTube about how the so called "free range" chicken are raised for consumption on the food market, over there I've seen this happening and it was disgusting, but I thought that was more like an isolated incident, didn't know that there is such "disease" among chickens.
I would've done the same, get rid of the cannibals.