altcoin and bitcoin prices flock together against fiat money. Part of it is most altcoins are denominated in btc so it passes thru declines in btc immediately to altcoin. however, most altcoins have a much higher price volatility than btc, so always expect them to swing the direction of btc, but with greater magnitude.

of course, if theres altcoin specific news item thats driving prices, that can overwhelm any effect btc is having on the altcoin and destroy correlations