My understanding from both Steven Van Metre and Cathie Wood is that a lot of banks are sitting on unrealized losses due to the unprecedented rate hikes.
But what caused started it with SVB was two things. SVB is highly concentrated with customers in Venture capital and their start up clients. Venture is perceived to have too much risk right now, so people were withdrawing their deposits.
And the bank was offering lower interest rates than money markets, so depositors were also shifting deposits from the bank to money markets.
This forced the bank to sell some of the assets that were underwater due to the rate hikes. When they did, they had to recognize a huge loss. And they tried to raise capital to get their capitalization ratio back up.
The market saw that and panicked.
Not sure what caused the other bank, but their clients were heavily crypto. And they were under some of the same interest rates are better elsewhere pressure that SVB was.
Here's a similar take; https://www.cnbc.com/2023/03/10/silicon-valley-bank-collapse-how-it-happened.html