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, it’s 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.
But being that a product can be the first of its kind to be made (just like Bitcoin) and other companies may decide to produce similar product;( just like alts) but that doesn’t mean the original product price and value will always affect others. So why does Bitcoin have such a big influence on other alternative cryptocurrencies.
The reason for Bitcoins becoming so popular is obvious. I mean any one who at first heard about Cryptos is only due to Bitcoins. From this you can measure the popularity of the coin comparing to other alts.
Also most of the alts follows Bitcoins in regards to its price is due to the trading pairs. Bitcoins are vastly used in many exchanges as a trading pair with other cryptos. So whenever there is price fall or rise in Bitcoins, it also followed by the other alts.
In other words many person do transaction on alts for Bitcoins only, so the similar pattern is seen.