When Microsoft first announced the Windows 10 May 2019 Update, it announced some changes to the way that it will deliver feature updates. The big one is that they'll no longer install automatically install new builds as they arrive; instead, you'll be prompted with a "download and install" option.
There's one condition where an update will automatically install though, and that's when your current version of Windows 10 is nearing the end of support. Version 1803, for example, will no longer be supported as of November 12, so Microsoft has confirmed that it's beginning the automatic rollout for those users.
Microsoft did say that it would begin automatically updating those on version 1803 in June, so the company is right on track. The Windows Update Twitter account said that it's training a machine learning algorithm, so that also means that it's not just going to send out the update to everyone at once. It should still be a staged rollout.
It's certainly worth noting, though, because according to the latest AdDuplex report, 61.1% of Windows 10 PCs are still running the year-old version of the OS. To find out what version you're on, you can go to Settings -> System -> About, or you can use the winver command.