I've spent 33 years in consumer and commercial banking. You are supposed to manage rate risk with short term duration bonds (90 day), hedges, etc. This is money you have to have ready on demand for withdrawal requests.
If you watch the video posted by Biostud above, it provides one reason why SVB failed (besides a concentrated customer base of tech startups vs mom and pop depositors):
They mentioned two types of holdings to invest customer deposits -
1)Bonds held for sale which are marked to market quarterly (they were forced to liquidate at a loss because the mix held longer term bonds which lost too much value)
2)Bonds held for investment which are longer term and kept on the books at face value until paid back in full.
The bank liquidated all the bonds held for sale and effectively lost over $1 billion in customer money. They had to raise capital to cover the loss. Unfortunately nobody came forth to buy the securities.
Overnight, people with millions of dollars at the bank realized they needed to get all their money out before everyone else causing a bank run. Now the bank was really screwed because they would have to liquidate all their bond holdings at a big loss.
The regulators knew that with no new capital and not enough money to cover all deposits (thank fractional reserve banking), THE BANK WAS TOAST. So they shut it down during working hours which is unheard of. They usually get shut down over a weekend.