Unfortunately, that's what dogs do. They're very 'nose' oriented--they use their noses more than we use our eyes to pick up signals about their environment. They 'read' scents like we would a medical report or a personal profile on a dating site...only they likely get more accurate information from their noses than we do from the internet!
About the only way to stop it is to train her to walk at heel all or part of the way, which is okay to a point, but she'll be much happier if you take a few breaks along the way to let her just be a dog and do some sniffing.