Your Mac can automatically download, but not launch, the macOS Sierra installer
Nobody likes waiting. That’s why I bet if you pop open the Mac App Store pane in System Preferences, I bet you have the box checked to “Download newly available updates in the background.”
Now, if you have that box checked, your Mac will download not just app updates and incremental OS updates, but major updates too, starting with the installer of macOS Sierra. This new behavior is rolling out over the next week to users of El Capitan versions 10.11.5 and 10.11.6.
Don’t worry. The Sierra installer isn’t going to just launch and update your Mac without intervention from you. You’ll get a notification from the App Store that it’s ready, just like you always have, but if you click that notification, you won’t have to wait for the installer to download. It’ll just launch and guide you through the process.