yea, I am just as baffled as you are... never seen this before.
No, there is no warning/error message.. the screen just goes black and then a screen saying windows did not shut down successfully etc.. then has the options of starting in safe mode, safe mode with networking, safe mode with command prompt or start windows normally highlighted and will start after so many seconds by itself unless you hit enter. I think today I will disable all add-on's and see if it still does it.. just a thought? I know all the stuff we have done has improved it a lot from where it was before.