It is because the altcoin is just following btc price not the btc following the altcoin. Because everytime btc rise altcoin will also rise and if btc fall the altcoin will also fall, it is always happening so this is not new to us and everytime it happens there is always a prediction and speculation but still no one can predict it.
It is true that altcoins price is following BTC price. That's why when BTC is increasing significantly after BTC halving, all altcoins will have a bullrun season too.