Im confused too why this things happen 
, if I remember last year when BTC pump altcoin dumps and when BTC dump altcoin pump , but right now there are moving in the same trend if BTC dump altcoin dump much more .
This question bothers me too. What i have noticed is when the price of bitcoin goes up , the price of altcoin goes down and when the price of bitcoin goes down the price of altcoin goes up. So i think its invesely proportional to the price of bitcoin. But sometimes the prices go oppposite. So there is no fixed equation to the proportionality of their prices., if I remember last year when BTC pump altcoin dumps and when BTC dump altcoin pump , but right now there are moving in the same trend if BTC dump altcoin dump much more .At the moment USD market very predictable if you watch BTC price,just compare graphs. Ive been trading with a formula keeping the relation between two prices the same and its been very accurate so far i cant complain.