Why chickens eat each other?

In the wild, a chicken would stay with the flock for protection. They wouldn't normally search out other animals to eat with, as that would be competition for food.

Chickens in a home flock will certainly eat with other animals, since the food is provided. Chickens are very useful ranging with cattle, sheep or goats, as they help reduce the insect population in the pasture. They keep pen and cage areas for rabbits and pigs cleaned of weeds and insects. Pet chickens have been shown to be quite tolerant of, and share living and eating space with, the other pets of the home.