Whenever bitcoin rally occurs,all altcoins starts bleeding.This is because due to altcoin holders just selling their altcoins to buy bitcoins.
When bitcoin price falls,thenaltcoin prices start to increase as many bitcoin holders sell their bitcoins and buy altcoins to increase their bitcoin volume.
Both are inter related to each other.
Honestly, it doesn't make sense if altcoins price decline and then people will sell it to buy bitcoin which is more expensive, cheaper altcoins for high-priced bitcoin? I will consider it as a big loss if sell altcoins at this point.
Usually, it's happening due to bitcoin price increase but altcoins price doesn't, which mathematically display red price, we can measure it by altcoins price worth in fiat currency.