It is due to loss of facial volume, drooping of the neck muscle, laxity in the muscle layer and loss of the connections between your neck muscles and the deeper structures within the null. The platysma is the muscle that wraps your neck and this is attached to the facial muscles. When you lose volume in the face from losing fat, tissues in your skin and the bones in your face getting smaller, this neck muscle descends. Also you lose the connections of your neck muscle with the underlying deep neck structures with time as well. I think that what also happens is that your posture is affected over the course of the years so that the cervical spine also becomes lax in such a way that the distance between your chin and cervical spine is shorter which contributes to the extra tissue between them. Also as you contract more of your neck muscle, this muscle shortens with time. The shortest distance between two points, your chin and lower neck, is a straight line.
Dr Young is located in Bellevue near Seattle, Washington