The problem is bonds are used as a hedge when the other markets turn to shit. They're a lot less valuable when inflation is high.

They have low interest rates but since governments, especially the US government (as long as the debt ceiling fuckery gets resolved), are very reliable debtors so there's almost no risk in getting paid back. Because the interest is a set amount over time; if you want to liquidate them quickly you'd need to sell them lower than whatever the remaining interest is. Profit margins are razor thin at the best of times.

Right now high inflation means the bond markets are crappy in general. So the SVB bonds were sold at a steep loss.