You would be able to unwind these positions gradually and the markets are relatively liquid for MBS and super liquid for T-Bonds. The problem is that you would

You would be able to unwind these positions gradually and the markets are relatively liquid for MBS and super liquid for T-Bonds. The problem is that you would be unwinding these positions after the interest rates have moved up, i.e. at a tremendous loss. Rates go up - bond prices go down and your assets are not worth as much. They did not want to do this right away and book a few billion in losses. It would be a hard decision for anyone to make. They bet that the rates will move lower soon but, instead, the rates went higher still. Now, if SVB could wait 10 years to maturity, everything would have been fine. But SVB is a bank and customers demanded money right now. They could not meet their obligations without immediate selling at a tremendous loss.