About an hour ago I got a system update on my Nexus 5. It still shows as Android 6.0 so it's minor fixes and adjustments. So far, I haven't noticed anything.
What is your security patch level? Google posted the factory images for the November patching yesterday, this may be what you got but as an OTA update.
November 1, 2015 I turned on my Nexus and got a message saying an update had been downloade. I clicked "install" and it did. I was on the screen with the green Android figure for awhile and apps were slowly loading. I have no idea what it was. My Nexus is working okay but it was before, too.