I am not an investment or banking guy. And I’m not sure what category of activity this type of lending falls into. But wouldn’t it make more sense to write off the investment losses rather than throw more bags of money on the burning pile?
They have capital ratios to maintain.
If the underlying assets (the assets backing the bank), move in value, then they need to provide extra capital from somewhere. This is them securing that capital base that they need due to the change in value of their current assets (largely US treasuries and mortgage back securities- this isn't really about the value of their tech portfolio).
The world is currently trying to meet the Basel III standard:
https://en.wikipedia.org/wiki/Basel_III
The situation in any specific country can be bit different in the timelines and ways they meet the standard however.
0 - https://www.federalreserve.gov/monetarypolicy/reservereq.htm
The fed, fdic and occ all regulate banks capital and have strict requirements around it. Not to mention their equity holders.