Germany has had exceptionally low bond yields for a long time. They actually dipped to minus 0.5% in 2019, which meant bond holders were paying the German government for the privilege of lending money to them.
This was not an accident. It stemmed from the German debt-brake, also known as the Balanced Budget Amendment, which was written into Germany's constitution in 2009, in the wake of the Great Financial Crash. It restricts the German government's structural deficit to no more of 0.35% of GDP. If the German govt wants to spend more, they must grow the economy first.
But this is now in jeopardy. The SPD who are part of the ruling coalition, want to get rid of it. The opposition CDU, say they are open to modifying it. The tiny Free Democrats (FDP) are the only ones who want to keep it, and as part of the ruling coalition, have blocked changes to it. But it's unlikely they'll be part of a governing coalition after the election.
The bond markets are reacting. For the first time ever 10 year German bund yields are above 10 year EU bonds.
A lot of the people advocating that Germany borrows more assume that "of course" Germany will be able to borrow cheaply because, well, it's Germany.
But the low interest rates were in response to Germany's financial discipline over a long time. If that goes, Germany becomes yet another country that is borrowing. Moreover, Germany will become a borrower with a demographic problem and low growth.