EDIT: Elsewhere in the comment thread people have clarified that the bonds are the big problem on the assets side. They had a bunch of low interest rate bonds that now have to be sold at a loss because interest rates are much higher.
Banks lend for long periods on the expectations that deposits that are withdrawn by some customers are balanced by new deposits from others.
But of course in the current environment there aren't a ton of startups depositing recently-raised funds.
They lost on multiple fronts, interest rates rose so much that it became a huge loss to sell these bonds, the tech market slowed down meaning that fewer companies were getting infusions of cash and more of them were burning through their cash piles and the final death knell was that word of all this got out and it started a bank run.
The first two are the same thing and the latter is not really an "event" but rather the legally-mandatory observation of the first two? (Unless they want to commit fraud, of course.)
ISTM the issue was that their core product was as shallow as their customers'.
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.