The jump seems to be sparked by the announcement by the finance ministry, in raising the coupon rate on 10-year Japanese government bonds to 0.50% from the previously 0.20% level. That makes it the highest level since December 2014.
The BOJ is continuing to conduct emergency bond buying operations in trying to pin down yields but they are being tested once again, after having tweaked their yield curve control policy last month. The jump in 10-year bond yields to 0.50% makes it the first time that the upper limit is hit since the central bank tweak and that is the highest level since July 2015.