It seems that some of Lumia 925 and Lumia 1020 devices have been facing a Windows Phone 8.1 freezing bug post Windows Phone 8.1 update. There is a thread over Microsoft forum, that has many users complaining about this issue. Now, a Microsoft employee has acknowledged this issue on their behalf and has confirmed that a fix may be coming to most of devices with “Lumia Denim” roll-out. Though he mentions, some devices may get it sooner.
I have both Lumia 1020 and Lumia 925 running Lumia Cyan and latest version of Windows Phone 8.1 update 1 (dev preview) with me and I have never faced this issue. So, Windows Phone 8.1 update 1 may bring the fix for this issue, when it rolls out in form of Lumia Denim update to affected devices.