Why am I no longer a 'Top Contributor'?

TC badges are awarded at the first of the week. They are based on your recent activity.

According to Y!A : "A "Top Contributor" is a member of the Yahoo Answers community who has shown that they are knowledgeable in a certain category. Each user can be a top contributor in a maximum three categories. You earn it or lose it depending on your recent participation in a certain section. The more positively you contribute, the more chance there is that you will get a TC badge. Just like featured users or category leaders, it's another way to give credit to community members."

Y!A has never said what the precise requirements are. It's commonly accepted that you need to answer a lot of questions (50?, 100?, who knows...) in a category and get Best Answers on a certain percent of them (probably 10-15%) each week.

You can have TC badges in up to 3 categories at a time. There is no TC badge for the Jokes & Riddles and Polls & Surveys categories.

:o)