It's fear. When everyone is making money, confidence is high in crypto institutions and pegs are strong. But when a major crypto institution like FTX (that was supposed to be as legit as the come) becomes insolvent after apparently squandering the deposited funds of users which weren't supposed to be invested in the first place, people start to think that maybe the party is over, and that maybe, just maybe, Tether doesn't have the funds to back up the USDT peg.