Both the linkage between the muscles and person’s attitude of supporting the body or body parts (I call this attitude “allowing bone-slant”) are related to this response.
The abdominal muscles are attached to the ribs, and the ribs form the thorax. Once the abdominal muscles are tightened, it pulls the thorax. If a person has the attitude of “allowing bone-slant”, which means that the person allows axial bones (including the spine and the skull) to move or do not try to keep them at a certain condition, then the person will move the thorax down by the pull of the abdominal muscles. Moving the thorax down means that the spine has flexed. This means that the skull, the head, has been moved and the position of the head is altered.
Let’s say the person didn’t move the thorax even if the abdominal muscles are tightened. Tightening the abdominal muscles often accompanies with tightening the sternocleidomustoids, one of the neck muscles, in the activity of supporting the axial body. You will see this if you tight your abs intentionally. You also feel the neck muscles (the front side of the neck) tightened. There often is a linkage (or co-occurence) between them in response. Thomas W. Myers mentioned the similar linkage called “Superficial front line” in “Anatomy trains”. His linkage is more physical (means direct linkage through the fascia and not in terms of response). The linkage I talked about is in our response. Therefore, the sternocleidomutoids are tightened by tightening the abs. Again, if the person has the attitude of allowing bone-slant, the person moves the head by tightening of the sternocleidomustoids. In this case the person’s lower part of the cervical spine has been flexed to some degree. By the way, the upper part of the cervical spine has been extended because the sternocleidomustoids attach to the back side of the skull. The head in turn is pulled down and back as well known condition in this case.
In either way it is very easy thing to happen that tightening the abdominal muscles alters the head position.
Regarding the attitude of “allowing bone-slant”, I explained it in this article below. This is the outline of my whole idea. It is a kind of long..