They are on a push to have everyone's contact info up-to-date and in a safe place in case of disasters.
After Katrina many JWs were missing and no one knew how to contact them. From what I've heard they never really figured out what happened to many of them.
I'd like to think that many took the oppurtunity to fade away. And it may be that the WTS thought of the same thing, so they came up with the idea of making sure that the elders have cell phone numbers so that they can contact everyone "to make sure everyone is okay".