Some exchanges don't even use anything but Bitcoin to change for smaller coins. So this is where the correlation come from, in order to invest in another coin, your investment Trail is going to be Fiat, to bitcoin, to other coins, back to bitcoin, and finally Fiat back out.
Also, people know that if they have funds they want to continue to trade with they are safe to turn those into Bitcoin and let them sit for a few days or even a week or two. These days all the altcoins are down and so for those who are in Neo / Bitcoin market has to wait until bitcoin price is settled and people start investing again in altcoins.