The purchase of 10 year bonds also implied a bet that faster maturing bonds won't be more valuable.
As shown in https://fred.stlouisfed.org/series/T10Y3M that is no longer a true statement and that bet failed. It was a true statement for about 15 years with one flirtation in August of 2019. It appears that this is is more than a flirtation and more of a dip than past events have been.
The bonds are as secure as ever - just that more money can be made faster in something other than the 10 year bonds.
If (and that's two letters with a lot of weight) we had continued the tech growth seen in the early part of the pandemic and money flowing into SVB, their plan would have worked (or worked better at least), but they failed to account for the possibility that interest rates would go up and that people would be hesitant to fund startups and the startups would be taking money out for payroll faster than they put it in from new rounds of funding.