Market Capitalization = Price * Circulating Supply
price is fixed at $1, even with fluctuations that USDT has in the recent past the price still stays around $1 with little difference.

this means the increase in Market Capitalization is caused by an increase in their Circulating Supply.
in other words they produced about 160 million new Tether tokens in the past 20 days.

it has nothing to do with people shifting to that altcoin!