It's a popular saying: If you want the golf ball to go up, you have to hit down on it. You make your best iron shots when the clubhead is still traveling slightly downward at contact and the club hits the bottom half of the ball. But many players make contact above the ball's equator, resulting in a low-flying or bouncing ball. This is called "topping" -- also "skulling" or "thinning" -- the ball, and there are several common reasons that it happens.
In the proper address position, you should bend at the hips and flex your knees. You might keep this position during your backswing but, as you start your downswing, you might straighten your legs and back. Perhaps you're trying to hit the ball harder. You might have slid your hips too far forward during the downswing and need to regain your balance. The ball might be in a bad lie, lower than your feet. In any case, standing up increases the distance from you to the ball; if you hit the ball at all, you probably are going to hit the top half.
Poor Ball Position
If the ball is too far forward in your stance, it's easy to hit it fat. The clubhead reaches the bottom of its swing arc before it gets to the ball. If you adjust your swing so you don't hit the ground, the clubhead will be swinging upward by the time it reaches the ball. When that happens, you have no chance of hitting the bottom half of the ball. You'll either top or whiff the ball.
Helping the Ball Up
Although the physics of golf demand that you hit down on the ball to make it go up, that line of thought is counterintuitive to most players. They want to help the ball up in the air, and the most logical way to do that is to swing upward. Unless the ball is sitting up on thick grass, which would give you a chance to get under it, you'll either swing up on the ball (as with poor ball position) or stand up during your downswing. But most of the time, you'll still top the ball.
Throwing the Clubhead
Sometimes you try to gain extra clubhead speed by flipping your wrists at impact, but all you do is straighten your wrists before the clubhead reaches the ball. That makes the club reach the bottom of its arc farther back in your stance, instead of in front of the ball. The clubhead then swings upward sooner than you planned and hits the top of the ball, not the bottom half. You not only top the shot, but also you create less clubhead speed than your normal swing would have.
If you lean toward the target at the top of your backswing, you'll tend to lean away from the target during your downswing. This changes the bottom of the swing arc. If you manage to avoid hitting the ground before you hit the ball (a fat shot), the clubhead will be swinging upward when it reaches the ball – and that's the recipe for a topped shot.