[snewman]

These bonds were supposed to be "safe" in the sense that they would be repaid on schedule. And that's been fine, AFAIK there is no suggestion that they won't be repaid.

The problem is that SVB, knowingly, took on "interest rate risk" by buying long-term bonds (average 6.2 years, I read) that lock in an interest rate. The money to purchase those bonds came from deposits, which can be withdrawn at any time. When everyone started withdrawing their money SVB had to sell the bonds, and they took a loss because interest rates have increased and the bonds they were selling were not longer worth as much.