This the reason that most people find the Modified Isosceles to be an effective stance. Before we do that, let's look at the traditional Isosceles stance. To get into this stance, you will want to place your feet about shoulder width apart, pointing toward the target. Give your knees a slight bend (nothing dramatic, you just don't want them "locked" in place). Next, you want to fully extend your arms.
This is the first place that people start to cheat themselves on the stance. They let their arms get sloppy. Instead of extending them straight out, they let their elbows sag a little bit. Part of effective marksmanship is gaining a consistent target alignment. If you fully extend your arms, that target alignment becomes a habit. If you do not fully extend your arms, then you have additional factors to consider in striving to attain good sight alignment later.
From this position (with arms fully extended and feet lined up side-by-side shoulder width apart) have a friend try pushing on your hands to simulate recoil. Now have them push a little harder. Did you notice your reaction? You no doubt felt a little off balance. You could feel the weight of your body pushing you backwards. Most likely you naturally took about a half-step back with one of your feet (typically the one on the same side as your shooting hand) in order to recover your balance.
Notice that this step was not a dramatic step backwards. In fact, if you are a right-handed shooter, you will probably notice that your right foot stepped back just enough that your right toes are lined up with your left heel. Holding this position, let your friend again apply the simulated recoil to your hands. Did you notice the dramatic difference you felt in balance?
Now we are going to add the final touch to our stance. This is where we get the weight of our body over the balls of our feet. The problem with this is that people try to overthink it (as with most parts of shooting) rather than just doing it. So here is a trick to get you doing it with very minimal thought.... stick your butt out! You accomplish this by bending at the hips. If you do this simple motion, you have naturally caused the weight of your body to lean over the balls of your feet. Now experiment and see how much more impact you are able to absorb. AMAZING!
The answer is simple; the brain HATES recoil. It doesn't like the movement and it can't stand the noise. So the brain, living in your head, tries to convince the head to move as far away from the sound as possible. But the eyes still want to see what is going on. So the head moves straight back. In fact, it typically moves a little further back with each subsequent shot.
Has this accomplished anything? In practical terms, NO. The arms are still the same distance from the head as they were in a good shooting stance. So the only thing that you have accomplished is ruining your ability to accurately sight in on your target, with the added bonus of not being able to control the recoil very well due to your unbalanced stance. Probably not the results you were hoping for.
So if you find yourself in this position, remember that you need to lean towards the target, not away from it. Extend your arms, take that half step back, and stick your butt out. You will be rocking this picture perfect stance in no time!