^ Hm... Last year around this time, the One Piece released the One Piece Memorial Best, a compilation of openings/endings (03/17/2010), in honour of the 10th Anniversary. It reached #1 on the Weekly Oricon charts with 82k in first-week sales (an uncommon feat for anime-related music albums) as well as #1 on the Monthly Oricon for March 2010 and then #10 for April 2010, had a decently long chart life (53+ weeks). It was the #28 album of the entire year, and has sold over 260k copies.

This year, a One Piece 3D film was recently released (03/19/2011). I imagine that might be the cause?