Traders in the crypto markets view stablecoins as a risk-off asset since they are pegged to the US Dollar or other fiat currencies. When traders look to take chips off the table, they sell their crypto for stablecoins.
Stablecoins typically trade very closely to their peg, but in extreme events such as the recent sell-off, the high buying pressure can make the prices of stablecoins fluctuate to levels way above the peg.