Pretty much all altcoins do not have a fiat pair (altcoin/USD). So what happens when the market turns red and people want to get into fiat to sit it out: they'll have to sell their alts for BTC in order to get the fiat. So when BTC/USD goes down, altcoins/BTC goes down and the dollar value of your altcoins will drop more than the dollar value of BTC.
This works in reverse too, when BTC/USD goes up steadily altcoins/BTC will go up and the dollar value of your altcoins will rise more than the dollar value of BTC.
The fact that the major volume goes to the tether pair which already faced the issue with its reserved funds. Real usd pair doesn't give any impact to the market. A lot of volume in the USDT pair makes everyone feels panic if the audit will prove if tether company just try to make a bullshit reserve fund to backed tether.