As a matter of physics (and ignoring air resistance): When you are no longer accelerating and are maintaining top speed, your foot has to land a little ahead of your center of mass. (Otherwise you'd fall on your nose.) As someone mentioned above, this is a braking force; you try to minimize it by having the foot moving backward as it lands.
Such a landing is easier to achieve if you lead with the knee, rather than with the foot. Also, some top sprinters have said that they had more of a sensation of their legs' going up and down, rather than forward and back, which may make it more natural to have a "pawing" motion on contact. (I was not a top sprinter and can't verify that personally.) For some people, emphasizing the backward movement of the elbows (during the armswing), rather than the forward swing of the hands, seems to help with the backward movement of the foot when it's landing.
But what really seems to help some folks is if they focus on brief ground contact--Jesse Owens's high school coach told him to pretend he was running on a hot griddle. If you touch-and-go, you'll probably have the foot moving backward on contact. Longer contact times make it too easy to have a passive landing, *followed* by a push of the foot; again, as also noted above, that puts more strain on the hamstring.
In other words, don't be so conscious of pawing back that you keep the foot on the ground for a long time. Touch and go, touch and go.