I think money market accounts don’t sell off old, but rather they hold short dated stuff to maturity. And constantly reset the rate they pay holders.

Of course, they can have counterparty risks. The counterparty is usually unprepared to actually pay up and expects to rollover its debt.

Sometimes that melts down and doesn’t happen:

https://globalnews.ca/news/160176/coventree-executives-faile...

(Scroll down to Canada in 08-09 here):

https://en.m.wikipedia.org/wiki/Asset-backed_commercial_pape...