There are options in macOS to automatically install various types of updates, labeled as 'app updates', 'macOS updates', 'system data files and security updates'. Maybe m6g6a has the last one checked.

I opened App Store and just had it waiting for me. Where the name would normally be in the Updates tab, it just reads "Install this update as soon as possible." [0]

[0] https://imgur.com/a/bMUKO

Edit: According to Apple's statement, they will automatically start installing the update on systems running 10.3.1 later today.