Why do some people look behind them....
It's not really turtling the head, it's keeping the shoulders up. In a proper isosceles stance, your shoulders are up. When you bring the gun in, you want to keep your shoulders up just in case you need to bring your gun back out to a firing position. It saves one movement of your body
Having said that, a lot of people use extremely exaggerated stances, and that adds to the potential silliness of what they are doing. My favorite exaggerated stance is the hunch back Isosceles stance. Yes, your knees should be bent. Yes, you should be leaning slightly over. But there's no need to bend your knees at an extreme angle, stick your butt out, and lean over at an aggressive angle. The advantage to it is you're a smaller target, but it's way harder to move around like that.