According to my own experience if btc drops, the same happens to most of alts, only a few are not affected, the best time to trade alts is when btc is stabilized, also traders buy stable coins when btc drops. Currently it exists a project called mintMe where traders and investors can deal with entrepenuers by buying custom tokens that belongs to a particular project, this tokens can be created at the same mintMe platform.
That's because of money coming into the market. In order to buy alts, you pretty much need Bitcoin. Sure, there's exchanges with USD listings, but you're still going to trade for Bitcoin then trade the Bitcoin to get the alts. This is why when you see the Bitcoin market cap up higher, all the alts also seem to be going up in price as well.
That's right, BTC dominance is still too high, so for trading alts we always must check out BTC graphs before buying any alts