The Windows 10 upgrade already automatically downloads on Windows 7 and 8 machines, but recently it's gotten even more aggressive. User reports have come in stating the upgrade attempts to automatically install without them initiating the process. You are still required to intervene to actually begin the installation, but this is still disconcerting.
Microsoft says this is a simple error that will be rectified, if it's not already: "As part of our effort to bring Windows 10 to existing genuine Windows 7 and Windows 8.1 customers, the Windows 10 upgrade may appear as an optional update in the Windows Update (WU) control panel. This is an intuitive and trusted place people go to find Recommended and Optional updates to Windows. In the recent Windows update, this option was checked as default; this was a mistake and we are removing the check."