Digital currencies are known to be more volatile than people think usually goes like this If USDT goes down BTC goes up, if BTC goes down USDT goes up Yes. prices for altcoins are soaring but maybe now it could be something else USDT is going up BTC is also going up and the word volatility is for the same reason.
USDT is pegged to dollar so it is not the USDT that goes down but the dollar itself. And when that happened since Bitcoin price is invesrsely proportional to US dollar, it is an obvious expectation that Bitcoin will goes up if US dollar has less purchasing power.
But if the USDT is depegged from the US dollar and keeps on crashing then the amount of USDT to buy Bitcoin will really increase.