Some causes for veins to protrude or bulge on the forehead are strenuous exercise and arteriovenous fistula. The reason for veins to bulge while exercising is due to an increase in arterial pressure that pushes fluid out of capillaries into cutaneous veins. Similarly, a bulging forehead vein can occur due to blood flow moving from an artery to a vein without going through a capillary, causing an arteriovenous fistula, states Merck Manuals.
Exercise, such as pumping iron, or other forms of strenuous activities, may be a main factor for bulging veins. When fluid rises up near the skin into cutaneous veins, these veins are more noticeable in areas that have less subcutaneous fat, notes Scientific American. The forehead is an area with little fat.
Arteriovenous fistulas can be caused by an injury or due to a congenital issue. Fistulas that cause veins that bulge on the face, can be purple in color, states Merck Manuals.