The Chromium team has released a rare third update to Chrome 65 in the Chrome OS Stable Channel today. The new build is 65.0.3325.209 (Platform version: 10323.67/68) and will be arriving on all systems over the course of the next few days. It is not uncommon for there to be two Stable Channel releases with each major build of Chrome OS. Usually the first one is for non-Android app running Chromebooks while the second, about a week later, brings the new build to Android running Chromebooks.
A third update however doesn’t happen often (it hasn’t happened since Chrome 62) and usually is reflective of the team finding a significant security flaw or bug that is impacting a high number of users.
As for the reason for this update, it is not known. The Chromium team did not provide any release notes or updates on why this was released and only provided the generic “This build contains a number of bug fixes and security updates” in the blog post about it. It could be that they will update notes later once a number of systems have been updated but that isn’t a given.
Regardless, if you have your Chromebook running in the Chrome OS Stable Channel, you can look for the update by typing chrome://help in the browser bar and checking for the update.