Causes
In general, cancer begins when an error (mutation) occurs in a cell's DNA. The mutation causes the cell to grow and divide at a rapid rate and to continue living when a normal cell would die. The accumulating cancerous cells form a tumor that can invade nearby structures. And cancer cells can break off from the tumor to spread throughout the body.
Gastroesophageal junction cancer is associated with having gastrointestinal reflux disease (GERD) and, less strongly, with obesity and smoking. GERD is a condition caused by frequent backflow of stomach acid into the esophagus.
There is a strong correlation between a diet high in smoked and salted foods and stomach cancer located in the main part of the stomach. As the use of refrigeration for preserving foods has increased around the world, the rates of stomach cancer have declined.
Source: http://www.mayoclinic.com