Unfortunately for some reason Google is currently rolling out Android 7.0 Nougat OTA for Nexus 6. The OTA will downgrade Nexus 6 from android 7.1.1 to Android 7.0 Nougat. Earlier, with Android 7.1.1 Nougat update Nexus 6 discover some bugs, this might be a reason behind this downgrade.
The New Android 7.0 update will broke Android Pay Support on the Nexus 6. Moreover, when you updated your device the apps on Nexus 6 will not work properly. you have to factory reset after downgrade Nexus 6 to Android 7.0 Nougat.
Google Community Manager:
There was a recent OTA update for Android 7.0 that is causing confusion for some Nexus 6 users. For those of you that want to guarantee future OTA updates, you will need to get back on the 7.0 supported track. If you are experiencing issues after accepting the 7.0 update, please factory reset your device to help resolve any problems. For any Nexus 6 users that want to continue flashing/sideloading future builds, you can stay on 7.1.1 and continue to do so.