I am still trying to get a handle myself on why chicks fight. Males do it most, but females can as well. It is a combination of genetics and environment. My games are really prone to do it and it can have lethal results. If I produce three broods from a given breeding pair, then one can have a major battle royal, another very little fighting, and the third no fighting at all. Having a mature male around appears to suppress fighting, yet broods with no older male may get along very well. Things that may promote fighting involve high temperature and / or even nutrition.