The value being in MBS now is a result of a separate transaction (well, a lot of them, probably) where (in aggregate) the bank takes a bunch of money from its cash assets and trades them for an (initially) equal value in MBS assets.
It becomes a problem when the MBS assets lose value, and the bank needs cash to cover withdrawals.
MBSs tend to remind people of '08 when MBSs lost value because too many had a bunch of garbage loans which defaulted, causing MBSs' face values to decrease. In this case, the fact that they were MBSs is mostly a coincidence. If SVB had invested in Treasuries, they'd be in the same world of hurt that they're in today.
Not necessarily, if they were investing/laddering in short term Treasury Bills (< 52 weeks) instead of longer dated bonds things might have panned out differently.
”97% of these MBS were 10+ year duration, with a weighted average yield of 1.56%.”
Treasury Bills from 6 months ago had yields of 3.37%.