SVB is unique in that most of its deposits were from tech companies, particularly startups.
Most startups are not profitable and need to drawdown their cash balance to pay for operations. When the economy is good, the successful startups get new cash injections from new rounds of funding and failed startups are replaced by freshly funded new startups, so the overall deposit base for SVB is stable.
In the last year, however, startup funding has completely dried up. For SVB, this means that all of its clients are drawing down their cash balance, without any new sources of deposits coming in. Even worse for SVB, recently many tech companies are having layoffs and so have been making even larger than usual cash withdrawals to pay for severances.
Ironically, there are rumors that many tech startups were planning layoffs last week and were forced to postpone the layoffs due to their inability to withdraw cash from SVB to pay severance.