From arstechnica.com:

[…] Microsoft has become progressively more aggressive. In the early days, this upgrade was found to be downloading the new operating system even before users opted in, and it was then accused of installing automatically.

Perfectly explained in this comic from sandraandwoo.com:

http://www.sandraandwoo.com/2016/06/09/0793-an-offer-you-cant-refuse-ww/