In my experience of Reddit you don't just get your subscribed subreddits, but also "you might like this" posts from subreddits you aren't subscribed to.
So I think there's definitely an algorithm at play here.
What happens (if you aren’t on old.reddit.com) is if you have too few subreddits it recommends stuff from other subreddits, and a lot of it is viral/outrageous stuff
What? Yes if you go to r/popular/ or r/all/ but on home page it wont (I just tested to be sure and it shows only my subscribed subreddits).