The reverse pivot is an issue that can creep up on you without noticing. The reverse pivot simply means that on the backswing instead of the weight staying behind the ball, the weight tends to move forward onto the front leg. This invariably moves the head forward as well and thus creating a move on the downswing that shifts the majority of weight on the back foot. This reason the weight comes back is that whatever you do on the backswing will typically have the opposite effect on the downswing as the body needs to counterbalance itself.
The result of the reverse pivot is typically 'thin' and 'fat' shots as the weight is staying too far back during the downswing. The way to fix the problem of the reverse pivot is firstly in the set-up. Make sure that the ball position is just forward of middle of the stance for the irons as well ensure the left hip and shoulder are lined up straight. This may require you to slightly bump your left hip to attain this position. Lastly ensure your head is just behind the ball with weight in your feet roughly 50-50.
During the swing you want to make sure that you move your left shoulder back behind the ball while keeping your left hip roughly in the same position. The left hip should never go forward toward the target but if anything come back a little around a couple of inches. You also want to keep the left knee quiet and your head behind the ball. Ultimately you want the weight to feel as though it can move back a little on the backswing rather than going forward.
On the downswing you want to feel that the weight is progressing onto the front foot. You want to feel that at the end of your swing that 100% of your weight is on the front foot with your body including your hips, chest and back foot pointing at the target, This will take a little while to feel depending on how long you have been reverse pivoting but over time it will start to feel natural.
If you'd like any further help in overcoming the reverse pivot please don't hesitate to ask.