Well, Dave Nevogt, author of "Simple Golf Swing", has written an article below on why we hit fat shots and how to fix it.
Here is Dave`s article...
Golf Fat Shot Fix
The cure for a fat shot...
This problem may be easier to fix than you think. You may not realize it, but your back shoulder may be dipping towards the ground. This move forces the club to hit the ground too early. Most of the time it happens when you are really trying to get into the ball. Imagine…You're 200 yards out and you've chosen to hit the 4 iron. When you've got it in your head that you have to hit the ball hard to get it there, it's natural tendency to start your downswing with the hips in an effort to "really get into the ball". That may work in baseball, but not in golf.