I am occasionally vexed by a hideous hitch in my left shoulder. What happens is that my left shoulder joint sometimes rises slightly upward at address (tension) and increases during the down swing, which exaggerates my shoulder tilt and causes all sorts of crazy shots--fat, pop-ups (especially with the driver), and thin shots. I've noticed it can creep in to my swing when I'm nervous or trying to belt one.
Not sure why it happens but it may be an unconscious effort to lift the ball during the swing.
And I'm not 100% certain how to cure myself other than to be aware of it; it's a sign of tension in my swing and address position.