Global bond jitters are spilling into Japan, a corner of the market that for decades experienced barely any volatility — and it’s worrying investors already spooked by frictions in US Treasuries.

Japan’s central bank—the Bank of Japan — seen as a “whale” of the domestic bond market because it owns more than half the nation’s sovereign notes, has been gradually trimming its balance sheet and scaling back its bond purchases. But the question is: Who else is interested in buying?