Windows 7 and Windows 8/8.1 users claim Windows 10 OS has begun to automatically install itself on their PCs. Microsoft has acknowledged the issue and reverted the checkbox, calling it “a mistake” Microsoft has been caught downloading the Windows 10 installer files to Windows 7/8 users, even without their knowledge. The issue actually resided in the Windows Update process. Microsoft said that the issue occurred with an optional update in Windows Update that was checked by default, calling the issue a “mistake”
Source: https://thehackernews.com/2015/10/windows-10-upgrade.html