when investors and traders see a coin's price rising amidst falling btc price they jump into altcoin to not ride the btc wave down. Buy altcoin low, ride the wave up and sell high.
Also, though many alts price in fiat drops as btc drops, many of the most successful alts btc price rises to maintain as close to steady fiat price. Some coins even grow in fiat price if those alts are gambled to be considered a safe haven for falling BTC price. I hope that makes sense to you as it does to me. Sometimes my explanations are not the most clear but I understand it clearly.