When a player shoots a gun, zombies will sometimes chase a random or nearest player to them if the shot is too far away. This also happens in the mod and always has. This is why sometimes zombies will come chasing you for no reason after you hear gun shots nearby. They chase nearest person instead of the source.
Thanks
Difficult to reproduce as it sometimes seems random. Easiest way to try reproducing this behavior is by having one player shoot on a roof to aggro the zombies while another player hides near the zombies being aggroed but far enough away and out of sight to not aggro. Sometimes when the zombies hear the shot, they immediately run to anyone nearby instead.