Similar to SVB, from what I understand. SVB parked several billion in treasuries when rates were at ~1.6%, when interest rates went up, that portfolio drew down, they exited with a loss, this triggered a run.
So if Circle has 70% of its reserves in T bills, they possibly took a market to market loss too, depending on how they were hedged.
(Disclaimer: i have not seen SVB balance sheet, nor Circles.)