Most people have Windows update setup to automatically install updates, and that usually includes Recommended updates. Now Microsoft is pushing the Windows 10 update as a Recommended update.
What this means is that your system may automatically update your machine to Windows 10 even if you don't want it.
The easiest way to block this is to run Windows Update and click Change Settings. From there, look for the Recommended Updates Section and make sure it is Unchecked. It means that you will have to occasionally check windows update for recommended updates that are NOT Windows 10 and install them.
See this article for More Information on blocking the Windows 10 Update.