Why Iphone Photos End Up on Google Photos? [Solved]
Further to other responses before, there is one more possibility that you have got Google Photos App installed on your iPhone.
If the app is installed ans running, it backs up your photos from your mobile to the Cloud. You will need physically log out of the app or to uninstall it completely for safe side if you are not a big fan of the Google Photos.
Personally, my every single photo on my mobile is backed up to Google Photos through this app. I just love the fact that it is free (up to 16MB and 1080p) and unlimited storage. At least for now anyway.
If you have a Google plus account on your phone you need to go into your settings and disable photo sharing to your Google account or disable camera on your Google account to prevent them from going to your Google upload automatically via cloud server.