Apple flipped a switch that makes it so your Mac can now automatically download its newest operating system, macOS Sierra. If you’re not a fan of this behavior, it’s easy to disable.
Sierra is now part of the App Store’s ability to automatically check for updates feature. This means it can download Sierra (but it won’t install it). Thankfully, the feature is smart enough that it won’t download Sierra if you’re low on storage or if your computer doesn’t support Sierra, but it’s still a bit annoying if you have no inclination to update.
You’ll need to disable automatic downloads across the board if you want to stop Sierra from downloading though. Head to System Preferences > App Store and then uncheck “Automatically check for updates.” Sierra isn’t nearly a big enough update for this to be a significant problem, but if you don’t have a lot of storage or you don’t have great internet speeds at home, a bulky5GB download is pretty obnoxious.