I'm getting notifications for subreddits that I've never been to before.

https://www.reddit.com/user/borg-assimilated/comments/1ieijyg/ive_never_been_to_the_radeon_sub_before/

I don't understand why I'm getting notifications for subreddits that I've never been to before or even knew about. How do I stop this? I looked at my notifications under settings and I don't see anything that would account for it.