From the Twitter support document "What's a timeline?"
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.
This is a new development, and takes Twitter a step closer to Facebook in terms of artificial manipulation of what posts you see. So far Twitter isn't saying "we don't show you all the posts from people you follow", just "we may also show you posts from people you don't follow if we think you'd find them interesting".
More discussion in a post by Dan Frommer at QZ.com.