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".