As with most problems with a golf swing, there could be a number of things causing it - ain't golf fun
With hitting fat, it seems that the biggest cause is not gettting a good weight shift. You wind up putting the right shoulder ahead which can not only give you a fat shot, but also in time injure your shoulder. Another big problem can be if you aren't accelerating in your swing when you hit the ball. If you consistently slow down at the bottom of your swing, you're going to see a lot of fat hits.
Hope this helps.