AFAIK there has to be some input from the user for the W10 upgrade process to go ahead; Tick a box & agree to the terms etc.
Sounds strange that it would be automatic as I thought you actually have to take up the offer.
Having said that a quick google indicates it's happening & people are pissed off.
Hmm!! That's a bit rude???
Apparently it can only happen if you have automatic updates set to install.
If you have updates set to notify, it will download the necessary Gb's of files before asking you to install it.
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