Can anyone shed some light on my my chickens were so vicious?

[Cluky]

Ok so one of my broodies hatched two eggs. I was ecstatic since one of the chicks was a porcelain d'uccle! Fast forward 8 weeks, chicks are bigger. All of a sudden yesterday the chickens started turning on the one chick! The chick had no wounds, marks or seemed sick. It was always a quieter one though. By mid day today they pecked it to death! Any idea what would make a free ranging flock go so crazy? Like I said chick was quiet but ate drank and was active. No wounds to get them going. Weird thing was that mom wasn't as protective of it past two days then before. I guess I am not so much looking as to what was in their heads as to try to prevent it in the future, it was horrible especially since kids saw it!