I had been struggling to see who exactly fired the first shot; but here it appears to be the brown shirt in a black toque and mask that's center screen for most of this video.
He watched the grey coat remove Alexs firearm, which means he knew he was shooting at an unarmed target.
The rest reacted to shots fired, unsure of where they came from; but still went WAY over board, shooting and continuing to shoot an unmoving target that posed no threat.