As far as I understand it, the auto upgrade happens when you have "give me recommended updates the same way I get important updates" checked in Windows Update. That isn't a default setting - you need to consent to it. Sure, maybe it was a year ago that you clicked it, and they never said a full OS version update was something that would land in that category, but it's not like they're pushing mandatory installs to everyone.
They just flash a dialogue up with an opt out button. Closing the dialogue is consent to go ahead with the update. Most people close those kind of annoying pop ups without reading the message. So technically MS aren't push a mandatory update - de-facto to millions of people they are though.
They pop up an information dlg telling you it will be upgraded. clicking x like close does nothing as it should. The lesson to learn is to not enable os auto updates, anywhere.
