Nashville Business Journalist editor, David Arnott, discovered this issue after uninstalling the app months ago before later deciding to give it another shot. After re-downloading the app, he found that the photos he had taken since deleting the app had been uploaded.
It’s been found that Google Photos' uploader settings are linked to the Google Settings app on Android phones rather than its own settings. These settings also handle things like Google's locations, backup and search settings.
It’s thought that this unusual situation is because Google Photos used to be a part of Google+.
If you once used Google Photos and don’t want your pics uploaded to the cloud, you’ll need to open the Google Settings app and turn off the "Google Photo Backup" option.