How do people develop esophageal varices?

Cirrhosis: When the liver becomes scarred (alcohol is the most common cause) blood can't flow through it to get back to the heart. The liver normally filters the blood coming from the GI tract. The blood still has to get back to the heart so it finds alternative pathways such as the veins in the esophagus. Over time these become swollen and can erode through the esophagus and bleed.

Cirrhosis: Scarring of the liver (cirrhosis) interferes with blood flow into the liver substance, especially venous blood coming from the gastrointestinal tract. This results in rerouting of this venous blood which is returned to the heart through esophageal varices, which are veins that become abnormally dilated in order to acommodate this increased volume of blood.