More of the altcoins are bought by exchanging bitcoin. Let's assume bitcoin as a piece of bread costing 12$ and you could buy any groceries in the market by exchanging bread. Suddenly if bread price went to 6$ then the amount of groceries you get would alter(decrease). Similar happens with the uptrend.
** Most importantly decrease in btc price might not make alts costlier the reverse also may happen because many people start dumping their btc to some alts and the supply of alts increases as a result it may become cheaper, similar happens when btc goes up.