Update: This update is limited to Australia, New Zealand, and India, and addresses carrier support / fixes in those regions.
If you're seeing an update notification on your Nexus 5, you're not alone - Google is pushing a maintenance release of Android 4.4.4 to the smartphone, though this one appears to limited to a few corners of the world. New Zealand, Australia, and India should be seeing the OTA, which contains a new radio image, likely addressing issues with networks in those regions.
KitKat MR2.2 (AKA 4.4.4_r2) OTA Update Rolling Out To Nexus 5 In Australia, India, And New Zealand, Probably Just Radio Fixes was written by the awesome team at Android Police.