I'm not a specialist in Japan however I got the impression that people there are quite conservative, no immigration, no children, work a lot, and buy a lot of their government debt, even though it's almost worthless and doesn't give any significant return.
It's very strange then to hear that Japanese people are buying BTC, it's counterintuitive.
It's not Japan or someone specific, when you feel yourself confident that you'll make a big Profiting through something surely you'll invest at least a small amount. The same is happening with the Japan people as the government itself has shown them the green signal. There is nothing strange Japan people buying BTC.
The real deal here is not only japan but the rest of the countries that accept bitcoin, we can not know where this pump exactly is from, but one things for sure there is someone or something that is pumping up the volume of bitcoin and If it will be great to bitcoin then I will humbly accept it, and the impending july 1 Australia will definitely join the force of pumping up the market cap.