I had a look see and here's the meat of an article that describes it:

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Now, after two weeks of TALKS with Microsoft, the company has admitted to me that the problem is very real…The symptoms follow two paths. For some they were using their computer as normal, were prompted for a restart due for standard updates and on reboot found the Window 10 installation was under way. For others they checked their Windows update history and found Windows 10 had tried but failed to install itself without permission over and over again. (screenshots at the link)

[...]

According to Microsoft the fault lies with a bug in an automatic update it began rolling out to Windows 7 and Windows 8 users in August which triggered the attempted installations. Microsoft didn’t disclose which specific update it was, but confirmed it left the error code ‘0×80240020’ (sometimes recorded as ‘080240020’) in Windows log files when upgrade attempts failed.

As the upgrade attempts all happened silently in the background, those affected would only know Windows 10 would attempt to update once they restarted their computer. For those computers where the update began but failed in the background (APPARENTLY a high proportion) they’d be none the wiser without checking their installation history between August and November.

[...]

So what’s the state of play now? Microsoft claims the bug has been fixed and there should be no further background attempts to install Windows 10 on unsuspecting users. The good news is those who contacted me have also confirmed the repeated installation attempts have now stopped.Of course the wider problem for Microsoft here is issue of crying wolf. Just last MONTH another bug in Windows Update for Windows 7 and Windows 8 automatically checked the optional tick box to upgrade PCs to Windows 10. Microsoft declared this ‘an accident’. (links at the site for more info)

http://www.forbes.com/sites/gordonkelly/2015/11/10/microsoft-admits-windows-10-automatic-installs/

According to Microsoft the fault lies with a bug