Why does gastritis cause diarrhea?

Various reasons...: The "gut" is all connected, by nerves, muscle fibers running inside the gut wall, and chemical signals (like hormones). If something is irritating one part of the gut (such as the stomach), changes can occur in other parts of the gut (the intestine). So, if stomach irritation leads to the intestine not re-absorbing digestive fluids or to it having faster transit time, there will be diarrhea.

May or may not: There are any agents that cause gastritis. Aspirin, alcohol, and some agents cause simple gastritis Which may be limited to the stomach. Many viruses will actually infect your gut from one end to the other, with initial features of gastritis which evolve into diarrhea as it moves down the tract. Once it triggers outporing of digestive water and the large intestines can't absorb it fast =diarrhea