Most of the altcoins are traded against Bitcoin or ETH but not all coins. So if bitcoin prices going up then people may start selling their altcoins to buy more bitcoin which can bring altcoin prices down. Even if bitcoin prices going down then people may start selling their altcoins in a panic that market may crash so during this time altcoin may fall even more. Only time altcoins will move up when bitcoin prices are mostly stable or less volatile.
I strongly agree with that! Being the father of cryptocurrency and being paired with alts leads to the volatility of price in the market. Only when BTC is up so does the other alts and may go down as well due to its flactuations.