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).