UST is an algorithmic stablecoin a.k.a. backed by the LUNA token. Hence it's pretty much almost backed by thin air; hence why Luna Foundation Guard had to purchase a good amount of bitcoin to protect the peg if it's the case that there's a bank run.

And yea, it happened:

Peg broke due to UST having a huge bank run, with people selling the UST for other stablecoins. Now for the peg to be restored to $1, Luna Foundation Guard had to liquidate a crap ton of bitcoin on their reserves just to push UST back to $1, fueling the downward pressure further