it has nothing to do with people shifting to that altcoin!
Yes it does. They produce Tether tokens when they can be backed by a dollar that they hold, and they can do that when the tokens are bought on exchanges (well they can produce when they like, but they're supposed to be backed).
I know that many people are shifting into USDT, but that doesnt increase the Market Cap of USDT.
Equally dumb. Yes it does. Increasing volatility, fear of an upcoming bear market and confusion about the weeks ahead is pushing people into USDT for the next few weeks.
Some big players are bringing the USD there - in my opinion
Big players almost never go into USDT. USDT is not safe due to the price fluctuations, and a Tether "cryptocurrency" does not work. They would convert back to fiat rather than be at the mercy of Tether.
When USDT becomes a "safe haven" in its current state, it really makes you think about the state of volatility right now, doesn't it?