Let me try to answer this, there are various decisions when Bitcoin price pumped

• A sell off their altcoins for BTC, so that they can ride Bitcoin movement to take profit
• B buy some altcoins with their USDT because these coins are in cheap price when paired to BTC, and end with the increasing demand of altcoins that make USDT pairs in a green color, later when Bitcoin is stable and BTC pairs will start increasing, B can sell their altcoins for BTC and gain Bitcoin more than if B bought BTC directly with USDT or Altcoins

I hope this makes sense