AT&T approved the Android 5.1 update for the first-gen Moto X (XT1058) on July 7th, but yesterday the carrier decided to halt the update. AT&T is calling this a "temporary" suspension, but isn't saying what exactly the issue is.
This OTA was a substantial jump from Android 4.4.4 to 5.1, so it's not surprising there could be some bugs. Although, Motorola had a long time to get this update sorted out. The take away is that if your Moto X is still on KitKat, it'll stay there for a while longer until AT&T and Motorola figure out what's going on with the update.
Details on the issue
Motorola's David Schuster has posted on Google+ to explain the reason for the hold. After a third of devices were updated, a small percentage of users had issues with missing contacts or syncing with AT&T Address Book. Motorola is working on a fix and hopes to have it ready soon.