The answer is that many altcoins are traded in BTC/ALTcoin pairs, not $$/ALT pairs.

So if an altcoin is trading 0.1 BTC/ALT, and the dollar price of bitcoin falls, then the $ price of the ALT will fall an equal amount.

Some altcoins trade against dollars as well, so their dollar price may be less coupled to the BTC dollar price. But for many of those alts, a large amount of the trading volume will still be from the BTC/ALT pair, and so the influence is still there.

I believe it all depends on consumer confidence. Bitcoin is the “Grand Daddy” of all crypto. It’s the original cryptocurrency and has the longest history. See this tweet by WhalePanda, a big player in the crypto world: https://twitter.com/WhalePanda/status/953206219703050240?s=17. This asset class is highly speculative so when people lose confidence in Bitcoin for any reason (such as negative news from China cracking down on crypto) then people panic and sell, so in fact, I would argue, it makes perfect sense. Confidence loss in Bitcoin affects all altcoins, as less established assets, for similar reasons. Cryptocurrency follows supply and demand economics so more people selling drives the price down. For those who have been in crypto for a long time, this is nothing new. Just look at the Bitcoin historical price chart. There has been several times of exponential price gains followed by significant drops. This panic sell off will pass as well and could be a great time to buy. It’s just hard to tell how far the price will fall. If you’re in Bitcoin you have to be able to stomach huge price swings.