Microsoft is doing this as a controlled gradual push, with more and more devices receiving the update as time goes on. Many companies choose to do things this way in case something goes wrong, then only a "few" devices get affected.
And it seems that this was the right choice, as people are reporting encountering major issues on some devices after receiving the update. According to said reports, there’s a specific issue on HTC devices, both the 8X and 8S, that freezes the device while playing music. We know GDR2 brings some improvements to Xbox Music so this might very well be the root of the problem. It seems like not even a full battery discharge can solve the problem, so some devices are getting completely bricked.
We haven’t had any problems with our updated phones and right now this only seems to be happening on a few devices. Hopefully Microsoft is working on a fix, and things get back to normal soon.
We've reached out to Microsoft for comment and will update this article when we hear back.
If you’re experiencing such issues with your updated phone make sure to let us know in the comments below.