The price moves with supply and demand on the secondary market. Normally, yes, that’ll tend to vary to balance yield with the prevailing interest rates, however, the threat seems to be to dump bonds onto the secondary market, presumably without a minimum price. The glut would mean buyers could purchase them below that balance price, giving them a better yield. This would have (at least) two knock on effects, firstly it would make it harder for governments yo raise funds through bond issues as they’d effectively be competing with the cheaper ‘dumped’ bonds and so would need to offer an equivalently high yield, and secondly may allow ‘undesirable’ governments or groups to amass significant amounts of European debt, which potentially gives them more political leverage than European governments might like.