You stopped too early

For a time, clicking the “X” in the upper right corner of the Windows 10 upgrade prompt window was interpreted as consent to upgrade to the new operating system. The typical expectation for all users is that clicking the “X” ends the program, and in fact, doing so was the only way to reject the free upgrade offer for most of the pop-ups notification’s existence. The same prompt could upgrade your system to Windows 10 without explicit consent if you left you computer on for an extended period, as well. PCWorld received hundreds of reader complaints about “forced” Windows 10 after Microsoft began using those tactics.

The scare quotes are there because there was always technically a way available to avoid the upgrade, but there are two distinct instances listed of users being upgraded without consent.