It Was Corruption

It turns out that the collapse of Silicon Valley Bank, or rather the need to bail out its large depositors was an artifact of systemic corruption within the entire Silicon Valley ecosystem.

Here is the money quote:

As conflicting theories for the bank’s meltdown swirled, commenters from across the ideological spectrum seemed to all agree on one thing: VCs’ responses to the crisis were shockingly unprofessional. Some criticized VC leadership for a panicked response; others characterized the pleas for speedy government intervention as the "ravings of idiots." The harshest critics accused VCs and startup executives of being "asleep at the switch.” They claimed SVB depositors were financially negligent, citing reports alleging that some VCs and startup founders had received personal benefits, such as 50-year mortgages, in exchange for keeping risky uninsured deposits with the bank. 

See also this Bloomberg article for a slightly more prosaic analysis.

The short version of this is that there was a corrupt relationship between venture capital firms and Silicon Valley Bank, and while this did not cause its downfall, it did result in the outsized impact on US banking and the FDIC.