my guess is that when you're holding an angle for a long time you start to draw your attention to things outside the scope, and when the enemy peeks you can't react in time. shaking the crosshair is done to reset your focus (at least for me it's like that)
I'm guessing it's more of a nervous tick or just muscle memory. He knows it's bad but he likely can't change it. I would definitely not try to copy it.