Because USDT was hit by issues that made it fall by 20% this week. Bitfinex's Tether (USDT) market capitalization fell from USD 2.7 billion to USD 2.2 billion last October. The decline in market capitalization was triggered by the controversial issue of USDT so far.
One of the main causes is because there is a massive sale of around USD 550 million tokens since the beginning of October. On October 16, the Tether returned 250 million tokens. Reportedly, BItfinex has transacted approximately USD 250 million in USDT to the Buy Tether Fund address.
Bitfinex then moved another USD 300 million in USDT to the Tether backup address. Continued on October 9, again sent USD 100 million, and another USD 200 million on October 14.
The most controversial, besides doubting the truth of Tether backed up with USD, is because the manager between Bitfinex and Tether is from the same management. The lack of clarity made the controversy even stronger. When people began searching, the issue developed into Bitfinex and Tether began to collapse.