Seeing tweets from people you don’t follow, and that aren’t retweets? That’s no longer an experiment, but a feature, Twitter says. A new change to the service is surfacing tweets from users people aren’t following, something Twitter suggests will make things “more relevant and interesting.” Twitter began rolling this behavior out to users late last week, though it was believed to be an experiment that only affected some users, and potentially for a limited time. It was also unclear what exactly Twitter was relying on to surface certain tweets to particular users.
Additionally, when we identify a Tweet, an account to follow, or other content that’s popular or relevant, we may add it to your timeline. This means you will sometimes see Tweets from accounts you don’t follow.
Twitter, in its help document