It's like this, bond yields on JGB's are practically zero therefore the only reason someone would buy a JGB is because they foresee absolutely no inflation and perhaps the yen reversing it's decline and strenghtening. Why would it do that ? Well, money could flow back into the country if investors feel that they want to invest in Japan. This is a huge gamble. Either investors go all in, doubling down on a losing bet that has not paid off in 20 years or they bail out. If bond creditors lose faith that Japan's massive stimulus will not make Japan a more productive nation then they may lose faith that they will ever be paid back in a valuable currency and they stop lending. That's when interest rates rise and the whole house of cards comes crashing down.