"Investors also tend to tap money markets during periods of market stress, not necessarily when yields are higher. Assets in money funds peaked at nearly half of the overall money supply in 2008. There was a similar spike in 2001, the aftermath of the dot-com bubble."

https://www.wsj.com/finance/investing/the-8-8-trillion-cash-pile-that-has-stock-market-bulls-salivating-0a1b4a8c?st=if6a7kl2hfn1tiz