How did Opal come across a dog named Winn Dixie?

In the opening chapter, India Opal Buloni, the ten-year-old protagonist of Because of Winn-Dixie, meets her future pet at a Winn-Dixie grocery store. The dog, clearly a stray, has wandered into the store and infuriated the store manager, who waves his arms around and threatens to call the pound.