2. The government decides that, oops, it printed too much money in 2020/21 and is causing inflation, so it raises rates very quickly.
3. New treasuries yield 4 or 5 times as much in interest as the ones from 1-2 years ago. Why would anyone want to buy those old treasuries near face value now?
4. The old treasuries decline 30-40% in present value. Oops, they're not so safe after all if you need your money back before maturity, which is often decades away.
5. A bunch of VCs decide they'd like their money back today, not in 20 years. A bank doesn't have it on hand, so it goes under.
6. Banks going under is bad look, so the government decides to inject massive liquidity into banks, now finding itself both tightening and loosening fiscal policy simultaneously.
SVB is certainly not blameless here, but the Fed's money printer and the government's wildly excessive stimulus has to be one of the worst policy errors since the 2007/08 financial crisis.