Swaying can cause this problem.

When you sway back in your backswing and you
do not sway forward during your downswing, your
clubhead bottoms out and begins to move inside
well behind the ball.  

This misaligns the clubface at impact.

It can also be caused by, or in combination with,
an out-to-in swingpath.

With the right tempo, let your lower body start the
downswing, keep your elbow tucked against your
side, and keep your left side firm through impact.

Transfer weight from top of swing diagonally
toward right of target, while letting right elbow
come down.