I've recently helped many people who found Windows 10 on their computers unexpectedly. So, no, you aren't imagining things and no you didn't accidentally click on the wrong thing...Here's what I've found:
"The Windows 10 upgrade is automatically being installed on many Windows 7 and 8.1 machines as a result of Microsoft changing the status of the upgrade to that of a "recommended" update. Windows 7 and 8.1 machines sold to home users are generally configured to automatically install these recommended updates."