Unfortunately the company didn't reveal the cause of the issue which has affected both HTC and Nokia Windows Phones.
Users have suspected the preview release of Skype to be the cause of the re-booting issue, as deleting the Skype for Windows Phone 8 app seems to rectify the problem. However these devices have also been facing issues with email syncing which Microsoft has not acknowledged to be part of the upcoming fix.
It is important that Microsoft stay a step ahead at this time if they want to continue the success of devices like the Nokia Lumia 920. We know that they are already working on an Apollo+ update for Windows Phone 8, but finishing annoying bugs will be a great motivator for early adopters to purchase more Windows Phones.
source - AllThingsD