This is based on the (fallacious) folk tale that crows form tribunals to judge and punish the bad behavior of a member of the flock. If the verdict goes against the defendant, that bird is killed (murdered) by the flock.

Maybe it's called a murder because people in the dark ages were put in "crow cages" to be pecked to death by them as a form of execution. unkindness of ravens. crows and ravens used to represent an essence of death and or witchcraft