Most common: Reverse pivoting. This is where you are putting your weight on you front foot during the takeaway. This can be slight or extreme doesn't matter. You should have at least 80% of your weight on the back foot at the top of your swing. If you don't, and you have 50-50 or 60% (front) - 40% (Back) or worse, when you start the transition to the downswing you either stay at 50-50, or even worse "reverse pivot" your weight to your back foot to 40-60. This causes the center of your pivot (the point at which we rotate our body around) to stay too far back behind the ball or even move backwards. This causes you to strike the ground behind the ball (2-3 inches if that is how much you are reverse pivoting).

Drill to solve this: Put your feet together and swing. You cannot reverse pivot. If you do you will fall down. This will put your center right over the ball and keep it there. Then begin spreading your feet apart a little and swing some more. Begin the swing with 50-50 at setup. As you go back, move to 20-80. Then at the top, start the transition to 50-50 and at impact, you should be at 80-20 or better. At the follow-through, you should be able to balance on your front foot (100-0).

There is a downside to this drill. Folks get too aggressive to move their weight forward and they "slide" their weight to the front foot causing their whole head and body to move toward the target. This will cause the center of their swing to be too far forward and cause "thin" shots that go very low.

Solution: Transition your weight to your front foot by simply rotating your hips and shoulders on to your front leg. Use your left leg as a balance like it is braced against a wall. It cannot go past the point it was when setup.

The next less common reason for fat shots: Using your upper-torso as a hammer. There is a tendency for your upper body to want to help your hands and arms hit the ball. You pick up your body on the takeaway and drop it back down at impact thinking that more is better - more mass attacking the ball will make it go further - and what's better then the entire body? Nothing. So let's just pound that ball with my whole body and that will make it go farther!

You are now in the desperation zone looking for power where there just ain't none. Raising and lowering the body can cause you to drop too far and lower the bottom of your arc causing the fat shot.

Solution. Keep the spine-angle the same throughout the swing. Have a partner stand in front of you and hold the grip of the club on your nose or bill of your cap. You should not go above or below that point through the swing.

One more cause that is about the same frequency as the torso-hammer: The back-leg accordion. You start your setup with flexed knees (good). On the takeaway, you begin to stand up on the right leg locking it in a straight position (bad). Then you do one of these: 1. reflex it to get back to where you were at setup, 2. Overextend your hands and arms to makeup for the raised distance, or 3. drop your upper-torso down (a variation of the hammer).

Solution: Keep the right leg flexed throughout the whole swing. Have a partner crouch behind you at setup and place a club behind your back knee keeping it from moving through the swing. That will give you the feeling of not standing on it during the takeaway. Another drill: Squat lower during the takeway. Don't do this to the extreme, just squat a little. You will be in the correct flexed position at the top of the swing. Now you can use your back leg as a push-off to help your hips turn through impact. When it was locked, it couldn't be used at all and you loose turning power. Think of a runner at the starting blocks with his front leg flexed at ready to explode at the gun. That is the golfers back leg flexed and ready to explode at the downswing.

Hi Greg,
I'm pretty sure its the right leg accordion! I certainly get a much sweeter shot when I just let the club swing through without trying to force it. Unfortunately I have a brain and that seems to get in the way sometimes, and tries to convince me that stuff I shouldn't attempt might just be possible. Right now, I am concentrating on keeping my head level, knees evenly flexed and trying to accomplish an easy swing. The last couple of rounds have been an improvement.
Then there's just my pitching , chipping and putting to sort out!

BR Ian