When Bitcoin's price increases, altcoins' prices follows. When Bitcoin's price decreases, the price of altcoins follows as well. This is happening because the price or value of all altcoins out there is linked to Bitcoin and not to fiat. If you could notice in all exchanges they have this so called "Market". Let us take ETH for instance. If you want to buy/sell ETH in BTC Market (ETH/BTC), the price is in terms of BTC (i.e. 1 ETH = 0.1 BTC). So if the value of 0.1 BTC now is $700 then 1 ETH = $700, after a day the value of 0.1 BTC went down to $600 then the value of 1 ETH would be $600 as well (saying that the conversion factor of 1 ETH = 0.1 BTC does not change).