Are you sure you didn't have any filters set up? I love being able to filter out the garbage in my timeline, lol. One of my favorite features that I now can't live without.
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No filters set up...I've never used them.
I don't know what causes it...maybe the same thing that happens when you select a person on Tweetbot. It shows some recent tweets but sometimes if you want to see even their most recent tweet, you need to select the button with their "54,294 tweets" (as an example).