More investigation showed that some of the inserted tweets had been quoted in posts that those friends did actually click like on, or were tweets that the actually-Liked tweets replied to, suggesting that the service’s process has a bug. While a Twitter spokesperson was not specific about the reason, they backed that interpretation up and told Engadget “This is not a test. This is a bug involving Likes that we’re working to fix. The issue is affecting numerous accounts.”
Whatever the issue is, if you’d like some control back — in lieu of Twitter offering a strictly chronological timeline — user Andy Baio has posted a search filter that “strips out all retweets, replies, media, links, and people you don’t follow.” You can easily access it at any time by just going to www.realtwitter.com.