Since all cryptocurrencies are linked to Bitcoin, most of the other cryptocurrencies fall following Bitcoin as the price of Bitcoin declines, which of course is a big drop. As for why it's a good time to invest in cryptocurrencies, you can get more coins at a lower cost because all currencies are falling.
I agree, when bitcoin prices decrease then altcoin prices will decrease, but when altcoin prices decrease then not necessarily the price of bitcoin decreases, but sometimes altcoin prices go up even though bitcoin prices decrease, even if it is rare.