Its no surprise here. Japan has already legalized bitcoin and almost all of the store in Japan is accepting bitcoin so it makes sense that they will lead in the trading volumes as well. Bitcoin trading is nothing new to the Japanese although it went downhill after the Mt. Gox incident but when they decided to legally adopt bitcoin, trading volume again rose even surpasses China and US and at one time have captured 50% of the day trading volumes. That's why they are leader together with China, South Korea and the US of A.