Google usually offers security updates for its devices for one year from the time it stops offering Android version updates. The Nexus 6 and Nexus 9 stopped getting Android version updates last October, after Android 7.1.1 update and the company had promised to offer security updates till October 2017.
The Nexus 5X and Nexus 6P will not get Android version updates from this September 2017 and the company had already announced that it will be offering security updates till September 2018. Now it has extended the support for these phones till November 2018. Google doesn’t say the reason for extending the support and is not extending support for any other devices.
If you don’t know, Google promises Pixel and Nexus devices Android version updates for at least 2 years from when the device first became available on the Google Store and Security updates for at least 3 years from when the device first became available on the Google Store, or at least 18 months from when the Google Store last sold the device, whichever is longer. After that, it doesn’t guarantee any updates.