There are many reasons why the veins in your legs may seem to ‘pop out’. You’re more likely to notice it when the veins are closer to the skin. Large, twisting, bulging veins are called varicose veins and indicate an underlying valve problem. Vein valves are flaps inside your veins that help prevent your blood from flowing backward. When you have blood flowing backward, it pools inside your veins, and causes varicose veins.