How does atrophic gastritis predispose to gastric carcinoma?

Good question. Atrophic gastritis is caused by chronic inflammation of the gastric mucosa with loss of gastric glandular cells and replacement by intestinal-type epithelium and intestinal metaplasia eventually occurs. Metaplasia precedes dysplasia which is precursor to cancer in many tumors. Overall, this is an adaption of mucosa to chronic inflammation that pushes this cell to replicate and eventually mutate.