The issue is that SVB took in a lot of deposits in 2021-2022, and locked them into 10 year bonds at 2.5% interest. Now in 2023, a lot of their customers are burning money (withdrawing funds) and aren't raising money (depositing funds), and so they need to start liquidating the 10 year bonds.
The issue is that the 2021 bonds are discounted because interest rates have grown since, and they're out some money unless they can hold the bonds to maturity. So they're raising money to bridge the deficit to allow themselves to hold more of the bonds.