Cause: Your right shoulder is moving forward instead of down.
Solution: The right shoulder has to stay back and low - not high and forward.
There is a reverse loop, from what most people do, at the top. It's a very natural motion that we all make with a baseball bat. The reason we do it with a baseball bat is that we balance the weight of the bat directly above our hands at the top and then as we begin to swing the meat of the bat lags behind creating this loop. In baseball we perceive the meat of the bat being above our hands even though through the strike it's not but then it is again after impact, so we just think of it always above the hands.
The golf club is no different. What I think is the biggest problem in golf is that people try to invert the club so that the clubhead is way down and the handle is up. This move destroys all natural motion in the swing.
The club never needs to be what I call "up-ended." The weight of the clubhead during a full speed swing will cause it to dip way down striking the ball.
You couldn't stop it if you wanted to. Attempting to up-end the club in order to "put" the clubface on the ball is the worst thing you can do. You must learn to truly swing the club and "let" the clubhead hit the ball, not "put" it there.
The design of the golf club is almost like magic. It wants to square up through impact. A good player just swings and has total confidence that the clubface will be there when they want it.
It's as if he couldn't care less where the ball goes but it goes where he wants. There will always be some degree of variable in each shot which is why it's a game of skill. Butch Harmon says, "... hit it like you're hitting into the ocean. You can't miss." Which is why so many people strike the ball effortlessly and well on the range - because they couldn't care less where it goes, they're just hitting it, and because of that carefree attitude the ball flies well, but once you try to steer it, your natural motion is gone and suddenly you can't make it go straight.
It's all about balance. The golf club isn't very heavy but we still want to balance it. The club should feel very light in your hands, almost like it's floating through the swing. It will feel like you're swinging with the butt end of the handle pointing at your feet through the entire swing - like a baseball bat, but somehow the clubhead will reach the ball without you even trying.
Your body will tell you one thing but you can see that your body is wrong, because the clubhead does reach the ball. After practicing this way you can easily tell the difference between "swinging" the club and up-ending it.