Why does top contributor bar disappear sometimes?

You have to maintain that balance of Best Answers to total answers in the category you had your TC badge in to keep the Top Contributor badge once you get it. The exact formula isn't publicized, but it's rumored we need to answer approx. 40 to maybe 50 questions in the one category within a week, and of those answers, get 10 to 12% best answers. You probably fell below that requirement.¸.•*´