But you are right, if we follow the logic of offer and demand, actually when btc goes down, eth should rise. This would be true if cryptos were the man trading factor. But it is not. When the price of bitcoin falls, it is because (amongst other reasons) people are selling their bitcoin. But they do not sell to buy other cryptos, they sell to get back their initial currency (generally speaking, fiat money, such as dollars, euros, etc).
I believe its because of the pairing Exchange Platform has. Most of the investors selling out because of panic uses BTC-USD, thus not making any changes to other coins.
if the investors in altcoins are panic selling and are in fact exiting to USD through bitcoin, then what you are saying is that the altcoin dumps caused bitcoin price to drop not the other way around! is that what you are trying to say?
because it makes sense that altcoin dumps cause a drop in bitcoin but i don't think it was the case here.