Quoting Apple’s own support document: Prevent your Mac from downloading updates in the background
You can set your App Store preferences to “Download newly available updates in the background”. If you’re using OS X El Capitan v10.11.5 or later, these updates include major new macOS versions, like macOS Sierra. Your Mac then notifies you when the updates are ready to install.
This is a not-good decision, Apple.
There is a difference between these two things:
I want to automatically download updates to my computer.
I want to automatically download a 5+ GB new version of the operating system.
And that difference is obvious to just about everyone.
There ought to be a separate option for the latter choice.
Copyright 2018 Timothy J. Luoma