Your home timeline is made up of the tweets from accounts you are following, but additionally, Twitter will identify tweets and accounts that are popular or relevant, and add them to your timeline. On the help page though, Twitter does not explain how it identifies popular content, or how it determines what content is relevant.
This comes soon after another experiment by Twitter, wherein the service started sharing favourited tweets to follower's timelines, the way a re-tweet is.
The goal, according to Twitter, is to make the timeline more "relevant and interesting":
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. We select each Tweet using a variety of signals, including how popular it is and how people in your network are interacting with it. Our goal is to make your home timeline even more relevant and interesting.
Judging by the kind of tweets we have been seeing, it looks more like an attempt at monetisation than engagement, but even if engagement is the sole end goal, it marks a big shift for Twitter.
That sounds a lot like Facebook's timeline, which most users know is regulated through a number of unidentified factors to try and show you only the things that Facebook 'thinks' are relevant.
For many users though, the unfiltered chronological timeline that Twitter presents is one of the best reasons to use the service - it helps present a much clearer picture of the conversations taking place on the Internet when compared to Facebook, where everything you see is presented based on your likes and clicks, and is presented to keep you 'engaged', instead of informed.