If you’re seeing tweets from people you don’t follow in your timeline it’s not a glitch but part of a new Twitter policy.
In addition to the basic, essential definition of a Twitter timeline—“all Tweets from those you have chosen to follow on Twitter”—plus retweets and ads, there’s a new section:
[From Twitter] 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.
In most cases, these seem to be tweets favorited, but not retweeted, by people you follow. That change has concerned some users: Will everything they favorite—even semi-private tweets—be shown to their followers? It seems the answer is no; Twitter is only looking to highlight popular or relevant content.
See Also: What is a Timeline? (via Twitter)