Probably has to do with playing against more RHPs and not being protected with favorable matchups. May just be the hitter he is vs RHP. This might be part of it, but when he was at his high point offensively this year, his OPS vs. LHP was slightly over 1.000 for the season. Now its at .834 for the season. His OPS vs. RHP has actually raised slightly from ~.650s (going from memory) to .689.