There are multiple reports* that Windows 10 is downloading without permission on Windows 7 & 8 systems.

http://www.theregister.co.uk/2015/09/10/windows_10_forced_download/

They are saying if you have windows updates enabled then it will download automatically without your permission. According to the article this is confirmed by Microsoft. Yes I know, difficult to believe.

*Edit: The multiple reports are all rewrites of the same (but usually reliable) source so far.