Mainly the altcoin and bitcoin price depends on their demands,which one's demand gets increases the price gets higher.Like you said bitcoin price increases the altcoin price will start to decrease which means when the bitcoin demands get increases there are many people started to buy which makes it more demandful coin when this takes place the demand of the other cryptocurrency normally decreases so the price also decreases.If whole world's people start to use digital currencies which makes it more demand so both price will get higher and higher as per their demand.