Many Reddit users faced an unexpected behavior from Windows Update when suddenly it forcibly and silently started the installation process for Windows 10 without any warning. This happens if KB3035583 is installed, which comes with the following description:
Update installs Get Windows 10 app in Windows 8.1 and Windows 7 SP1
Without any confirmation, the updated Get Windows 10 app not only starts setup but upgrades the user's installed operating system to Windows 10!