Look at Basel III and their liquidity tests. TLDR: bonds 5+ years are considered illiquid, and banks which are subject to Basel III (which SVB would be under old rules) must have at least 100% liquidity coverage ratio (which based on SVB's client base would require SVB to hold ~30% in short-term bonds).

Edit: this FT opine summarize the sillyness: https://www.ft.com/content/c95e7708-b903-405d-a017-963844eb3... (paywall bypass: http://archive.today/2023.03.11-083455/https://www.ft.com/co...)