imo it's the other way around, other currencies that launch up every other day get investors who send their btc exchange for that coin. Which result in bitcoin price decline. Bitcoin gets massive hit from sell off from alt currencies investors.
When the market is turning into the bearish trend and there will be massive selling to the crypto currency especially altcoin. People are thinking if they can take the advantage by holding bitcoin and waiting for the price of altcoin to be declined a lot and they can re-enter to the market again. Bitcoin is dominating all of cryptocurrencies.