Ars Technica is reporting:
For the first year of its availability, Windows 10 is available for free to most Windows 7 and 8 users, and Microsoft has been trying to coax those users to make the switch by delivering the operating system through Windows Update. Until now, the OS has been delivered as an optional update; while Windows Update gives it prominent positioning, it shouldn't be installed automatically. This system has already generated some complaints, as Windows Update will download the sizeable operating system installer even if you don't intend to upgrade any time soon, but, over the last couple of days, the situation seems to have become a little more aggressive. We've received a number of reports that people's systems are not merely downloading the installer but actually starting it up.
I have seen this first hand. They are doing it by making it an "optional" update that gets checked by default. And not only do they check it by default, they bring it back and check it again, even if you tell Windows Update to hide the update.