Top Contributor Requirements.?

Top contributor badge is given on the basis of consistent participation scoring some minimum points each week. Once you relax in it, you may lose the top contributor badge even if you rank # 1 in overall or cateegory ranking. I do not know what the cut off is but someone in the past had given it to be 20 points a day or 140 points a week in a specific category. Since you are mostly in mathematics category, try to get at least 200 points each week in mathematics questions and you shall be marked top contributor in mathematics in 2-3 weeks. If you have the top contributor badge and you fail to maintain consistent participation, the badge is lost till you again become an active participant.

I used have top contributor badge for mathematics and physics, but lost the physiscs badge as my participation in that category had fallen below the cut off.

The Global and countrywise overall rank is given on the basis of total number of points. Since you have a large number of points, you must be high up in the overall ranking. Categoriwise rank is given only countrywise and not golbally on the basis of total number of best answers in the category.

Well, looking at your best answers, you haven't gotten once since page 3 of your answers. This could be why because supposibily you need a certain amount of best answers per week. You seem to answer a good amount of questions this week so that shouldn't be why.

You have to keep constantly contributing or you get tossed out.

It's not just about point total but something to do with daily or weekly average.