It's probably still nice to actually know what is happening. At least you switched it on in this case.

pro tip:

click on https://www.reddit.com/subreddits/

then open the console in your browser and type this :

     setInterval("$('a.option.active.remove.login-required').first().click()",400)

I didn't find the option to remove all default subscriptions at once in the settings, it might exist though. setInterval is needed because for some reasons ajax communication is throttled by reddit.