In a word: reflux. The blood is going the wrong way.
Arteries carry blood from the heart to the organs and the arms and legs. Veins carry blood back to the heart. In between are tiny capillaries.
Blood in the arteries is driven by the pumping of the heart. After blood travels through the capillaries, there is no “push” to carry blood further. So the human body has come up with an ingenious way to keep blood moving in the right direction: veins have one-way valves. Every time you contract the muscles of your leg, blood is pushed up to the heart through as the valves open. And when you’re not moving, the valves close to keep blood from moving back down. Over time, this system is good enough for blood to work its way back to the heart.
When these valves get damaged for whatever reason, blood pools in the veins, and the veins and their branches expand giving the appearance of bulging veins under the skin.
And that, my friends, is why people get varicose veins.