I don't know if "the experts" all agree with "this expert" but he pointed out that in buying all the mortgage backed securities, SVB got to put them in a "hold to maturity" account that is not "marked to market" (the value in the account does not match their decreased market value, so on the books it looks like a lot of value in there, but in reality it's not)
at the same time, as the coupons on these bonds pay out and the bank collects them, that money just shows up on the bank's income statement, not landing in the hold to maturity account itself as part of the value of the asset. This rings all the "hey, look we are profitable" and "hey don't I get a bonus" bells, twisting incentives for the bank and its officers (and other banks) to keep do this, lather rinse repeat.
recipe for disaster when interest rates go up.