I had endoscopy. I have gastritis and mild duodenal villi blunting. Some bile was found in stomach. No celiac disease. What could cause this?

Many things. Gastritis is inflammaton of your stomach lining and can be caused by chemicals such as bile reflux or medications such as aspirin, nonsteroidal antiinflammatories and some antibiotics like doxcycline. It is also associated with some bacterial infections like helicobacter. Villous blunting can cause malabsorption and can be caused by bacteria li as well as immune causes.
Unclear. There are many potential causes of gastritis, including bacterial infection and medications. The other findings, duodenal villi blunting and bile in the stomach are nonspecific. Regardless, the only way to sort out what is causing these problems in you is to follow up with your endoscopist. It is very likely that he took biopsies or other tests to try to sort this out. Good luck.