Very easy to explain. The money that outflows from BTC, when the price goes down, has to go somewhere. thats usually the alt markets, most people are not will to go through the costs and hassle of converting to fiat every time. So during this downtrend, the market share that is leaving btc is essentially causing this. this isnt a bad thing, btc will not survive by itself. but this is temporary for the short term. btc is still the most media friendly coin, and is thus the most promoted. this will change soon, when corps that have icos realize that btc spends zero dollars on marketing it would be really easy to make a larger splash on the public with a different coin, if only someone tried.