My guess would be that you're on the cusp of the top posters list and that another member posted a bit more actively than you over the past two days
and that caused them to move one notch higher than you on that list - thus bumping you down from #100 ( or whatever the cut off is ) to #101.
It's the new and improved "clear" color border.
Only I have it but I've given it out to all members without silver or gold.
That way they don't feel left out.
The clear border is pure awesomeness and it basically tells everyone that I won the internet...