Why it happens:
You either take the club back too far inside the target line and return it to the ball on a similar path (like a merry-go-round), or you take it back correctly but slide forward too aggressively during the downswing, so the club drops behind you onto a shallow plane.

KEEP YOUR HEAD STILL
You might have heard of the Stack & Tilt swing method. It emphasizes a slight increase of weight onto your front foot as you make the backswing. Ben Hogan employed this move in an effort to prevent his swing from being too shallow at impact and thinning the ball. Notice above how my left shoulder remains over the ball rather than moving behind it? This is a move you should try to achieve to create a far better impact position.