This is what happened in Japan causing Japanese bonds, JGBs, to collapse over night:
1 insurance company decided to dump all JGBs today against the expected BOJ-purchases today. When they went to sell, something went wrong and BOJ weren't buying. It was likely a technical or similar problem. Basically the BOJ all of a sudden left the market. There were no other buyers.
Japanese bonds are traded on an exchange and in an almost unprecedented move, the exchange had to close twice to sort this out. Almost never happened before.
This is fairly catastrophic news for BOJ, Abe and Japan. Even just a slight bump for yields is enough to bankrupt the country.
And if you are one of the many Japanese banks loaded up with JGBs, you have to imagine what they are thinking right now. I'm guessing it sounds like "Oh, Ship".