Someone who is betting that the negative yield changes favourably. The bet is the same in positive yield regimes as well. This could happen due to a combination of deflation forecasts increasing and expectations of interest rates being lowered even further.
The issuer of debt is the seller of the bond.
That doesn't the answer the question of "who is buying these bonds instead of US Treasuries?"
Someone who is betting that the negative yield changes favourably. The bet is the same in positive yield regimes as well. This could happen due to a combination of deflation forecasts increasing and expectations of interest rates being lowered even further.