When you get headshots, they are instantly executed. I don't know how many headshots are required for that, but I'm sure it's because of that.
It's not actually an instant kill, it's just that the same burst that downs them also breaks open their dome before they hit the ground. Also, even one entire burst to the head isn't enough for a down+kill. It's something like an entire burst to the head, plus one shot from the second, and the second shot in the second burst breaks their head open.
Are you ready? This secret will change your life, and how you think as a whole. Ready?
Semi Auto isn't the only gun in the game.
Of course it isn't. I regularly see, burst rifles, revolvers, 9mms, bows, and assault rifles. This is not a secret.