Can you feel it? It's in the air. Of course I'm referring to the impending arrival of Android 5.0 on Nexus devices. It's starting now with a full jump to Lollipop for devices that are currently on KitKat, and a small bug fix update for new devices like the Nexus 9.
As with all Google updates, this one will be rolling out gradually over the next few days. We should have the OTA URL at some point so you can update manually, but you're going to be waiting a while for the mobile data versions of the Nexus 7. Those updates will take a little longer. Likewise, the Nexus 4 update has not started yet.
When the update ZIPs are found, you'll need to have an unlocked bootloader or be willing to use ADB to apply the update manually. If you're on a developer preview or a custom ROM, you need to get back to completely stock Android to get this update.