I think that's because of the link that some altcoins have with bitcoin, when the bitcoin goes up you can see many people selling their altcoins not to miss the rise of bitcoin, you can also see the opposite if some altcoin goes up, but it is much less noticeable because it would be 1vs1 and not all vs 1 as it happens with bitcoin, you have to remember that most people change from altcoin to bitcoin and from bitcoin to fiat if they want to make another investment