Over the years, I have noticed that every time Bitcoin's price goes down other alt coins follow suite. And When the price of bitcoin increases, its expected that altcoins go up in price shortly after. Presently as bitcoin price reduced other altcoin prices was affected too. Fact is that Bitcoin has the most secured network and it was the first among others. No doubt some alt coins was made of bitcoins clone.
It is primarily because of the traders rather than the hodlers.
They group all cryptocurrencies in one category, thus when one big cryptocurrency makes very high gains (e.g. BTC), they buy up the other big cryptocurrencies, and it has a kind of bull effect on the market.