Every time I open Skyrim the launcher downloads mods that I previously subscribed to and gives me a message that says, "Synchronization complete. Some mods that were unsubcribed were subscribed to and loaded".
For all of these mods, I have (a) checked that I'm not subscribed to that mod on the Steam Workshop and (b) checked I don't have the mod file in my skyrim data folder. I can delete everything in the Data Files tab, unsubcribe from everything at the same time, go to the website and check I am subscribed to nothing, and the next time I open Skyrim it redownloads all the mods I don't want.