I think the Bank of Japan have really pulled back their purchases of long term bonds so more supply causes yields to go up.

For years the Bank of Japan has capped yields (yield curve control), to keep borrowing costs really low. I think they have really stepped back from doing this and it means long bonds are catching up fast to reflect inflation and reality.

I guess people are saying "if you want my money for 40 years, you will have to pay me a lot more for it." Investors no longer trust the government to keep inflation low or debt under control for decades, so they are demanding higher returns.

People watch this as it could signal the end of the cheap money in Japan...they get concerned it could cause a big unravelling event.