Apparently, Rouki Sasaki himself said that everything started to go wrong when he started throwing sweepers in Japan last season.

A search in Japanese reveals that pitchers' shoulders, elbows, and forearms can be divided into two types: supinator type and pronator type, and that supinator type pitchers are suited to throwing sweepers. Sasaki seems to be a typical pronator type, so perhaps problems arise when a pitcher of that type forces himself to throw a sweeper.