Probably the anniversary update. Takes a little while. Windows 10 will automatically update past a certain point. It doesn't just ask like previous versions did. The best thing you can do is select times of day you won't allow this to happen.
there's that. A new Build version was released around July. Then a new "Windows Updates" paradigm was announced for Win 7, and it seems to be effective for the October updates. Both OS's had massive updates through a couple roll-ups this month.
I had two overclocked systems get borked this month after the updates, but nightly backups on a WHS server saved my ass for about the fourth time in a few years from human error and drive failure, and a fifth time it made transferring to a new hard disk equally easy for a machine that just needed more capacity. Both Sandy Bridge K; both with the same motherboard and PSU; different but all-NVidia graphics setup; same RAM, slightly different capacity.
Some folks in the OS forum were telling me that "no OC'd system is stable," but one of these had been running for 5 years 24/7/365, and the other for 2 years waking, sleeping, hibernating daily. It wasn't the clock settings, nor was it the hardware.
I tracked the problem sources to the latest Silverlight Update download, and a problem with Kasperky IS 2016 that caused delays and possibly borked Win Update installs from user impatience, even with a veteran like me. The solution for people who had been complaining since July (the anniversary update) was to simply uninstall and then reinstall KIS.
Of course, this was primarily a problem that surfaced in Win 7, but the same symptoms were observed with Win 10.
There may be other reasons, because hardware, software, drivers, graphics -- everyone has a different situation.
I just got all this ironed out now. I could reinstall KIS, but figure to be prisoner of the company town and company store: I'm going to try Microsoft Security Essentials for a while. Maybe that's what Microsoft wants. Who knows?