macOS update closed all my applications?

At exactly 4 a.m., macOS gave a popup saying, “Update Available,” then abruptly closed all my applications. I don’t remember receiving any earlier popups or warnings about updates being available.

Is this normal behaviour? Can I prevent it from happening again somehow?

Having this happen at exactly 4 a.m. seems significant. Maybe it assumes I’m not active at that time?

You will normally be prompted to install updates as they are detected. However if you were away from your computer you may not see those notifications. Also if you delay them enough times they will eventually be installed for you. Lastly if the updates are detected overnight they may be installed right then as well. This is usually done sometime after 3 a.m.

You can go to  → System Preferences → App Store and uncheck the following:

• Install app updates
• Install macOS updates
• Install system data files and security updates

This will keep macOS from installing updates, quitting apps and/or restarting your system without asking you first.

You will still be notified when updates become available, and offered the choice to install them.