When bitcoin falls in many cases most alt-coins fall too. What I am going to say is not a science but more of a general rule of thumb.
Bitcoin is like dollar is to oil and many other products. We say that the dollar is the “reserve currency” of the world. This is because the dollar (we may question how long this will last ) currently is the most powerful currency on earth.
The same narrative applies to crypto currency. Bitcoin is the first and the most “powerful” crypto currency and is in that sense the “reserve currency” of the crypto market.
When you look at the price of an altcoin in dollars, you are indirectly looking at the price of that alt coin in relation to the exchange rate of bitcoin with USD.
So since most alt-coins are traded heavily against and somewhat related to bitcoin, when bitcoin prices in USD fall most alt-coins who are prices in bitcoin will also follow.
Now there are also situations when bitcoin holders move bitcoin into some altcoins, this is when we see some alt coins rise while bitcoin falls. We saw that last Wednesday where bitcoin was falling but ADA was surging. Some argued that this was because bitcoin’s energy issues were addressed by Elon Musk, what might have caused some bitcoin positions to be sold and converted into the much environmentally friendly altcoin.
To conclude, in our global economy, once every about 70 years a new currency/country becomes the most powerful. This is when the reserve currency position will change. In the 18e century the Gulden was the most powerful (reserve) currency (the age of the powerfull Dutch empire). Later on the brittish empire overtook the dutch and the Pound became the world currency. Because of the 2nd world war, England lost a lot of power and the USA and the dollar took over. So maybe in 10 years the Yuan is the world reserve currency and is ADA the crypto reserve currency