Is that the last app that was open or does the phone have any sort of shortcut to the camera app (like 'double tap the back to open X app' or a button press perhaps)? When you say 'touched', is that touching the screen, tapping, placing it on a table, etc.? You could test this as well on 'Safe Mode' to see if any 3rd party app or shortcut was causing this.
If this isn't a software feature you can turn off, by the way, I'd just go ahead and exchange this under warranty. You shouldn't have this type of issue on a new device lest it be a feature you didn't know about.