Twitter has made a subtle change to their timeline where users can now expect to potentially see content not just from people that they've deliberately decided to follow, but also content from people that Twitter's algorithm decides to show them as well as peoples' favorites. This may add some value in that discovering new content that users might be interested in could occur more frequently. This is what Twitter definitely wants. But this could also harm the integrity of the timeline and take away the perception from people that they're really curating the content that they see. It could also be detrimental to the user-experience for the average intelligent user if a bunch of Justin Bieber's posts now spread their way around Twitter even further!
Twitter's new approach, which also includes promoted tweets, is much more similar to how Facebook's own Newsfeed works. Opinions seem to be mixed, but many users strongly dislike the idea of this particular change. Twitter hasn't been shy about making small or large changes to the site before. From adding an official retweet feature to adding conversations, lists, images, and cards, Twitter has continuously innovated from a user-interface and user-experience perspective. Since this move is no longer just a beta or pilot program, but actually an official policy of Twitter, you can expect to see a whole lot more content in your Twitter timeline going forward.