Sorry for the bump but this happened to me again this week. I am running macOS 10.13.6 and noticed that Security Update 2018-003 had been downloaded. I did not run it at once. The next day after shutting down and trying to start up the installation log appeared just as it did for people postponing the 10.13.4 update. So whatever the underlying problem is, it continues.
Edited to clarify. Here are my App Store preferences:
My complaint is not that the security update tried to install automatically, because my preferences allow security updates to be installed; my complaint is that it failed with a very confusing notification.