They’re backed by treasuries and subject to treasury market shocks (like the March 2020’s dash for cash). Large redemptions can see a feedback loop of redemption -> rushed selling -> treasury market stress -> redemption. Exactly the sort of scenario one might expect as the secular trend of the weakening dollar bubbles to the market’s surface.

Stablecoins hold a sizable portion of the treasury market - https://fintelegram.com/stablecoins-became-a-top-20-us-debt-...