Altcoins have always on the whole followed the BTC price but with higher variance. So if you bought a basket of altcoins, I'd expect it to behave similarly to a leveraged BTC position; when BTC rises, altcoins rise more and the BTC dominance decreases, and when BTC falls, altcoins fall more and the BTC dominance increases.
Read this first.^
And sometimes, market cap is just meant to ignored, and you're ought to focus on how to profit, even in a bearish market. Because one man's loss is another man's profit.
(pretty sure I ripped off this saying)Bitcoin isn't making the altcoins slide. The fact that most altcoins are scams or trash is what is making altcoins slide. Most altcoins don't even have a working product - they aren't worth a thousand dollars, let alone the millions or even billions some of them are valued at.
Bitcoin isn't making the altcoins slide, true but the existence is. People obviously prefer bitcoin to altcoins, so when markets crash, they panic sell everything to bitcoin, "to be on the safer side".
Both staments are true as people tend to overreact if the market suddenly crash, but long time users and enthusiast know that even in the highs and lows of any market what an individual need to do is to focus how to make a profit for him self other than sulking on the status of it.
Also for people who sells alcoins to bitcoin is very much understandable as bitcoin is already established and is much more trusted which can be compared to USD in the stock or forex market which is widely used for transactions.