They made a bet that people wouldn't need their money back right now. It's the constant risk in fractional reserve banking.
SVB should have purchased more swaps or swaptions, but this is me Monday morning quarterbacking. If depositors had left their money like It's a Wonderful Life things would have been fine.
It's not that they were holding bad assets, it's that they put too much money into assets that took too long to mature. They were betting that they wouldn't need that money sooner than that, and that's the bet they lost.
SVB should have purchased more swaps or swaptions, but this is me Monday morning quarterbacking. If depositors had left their money like It's a Wonderful Life things would have been fine.