Listen dude, the basic answer of your question is that, all the coins which can be considered as Assets follows the trade of Supply And Demand.
If the demand will be more then the supply will be less and this leads to the increase of price.
And as BTC price is rising people are buying it in more demand and also comparatively some are selling the altcoins in order to hold some Bitcoins and we can say for this reason also the price is dropping.
Hope this helps you mate to understand the market with more depth.