Altcoins are purchased by morons. Morons that are trying to get more bitcoins. If bitcoins drop, so do altcoins because these morons are really bad at math and everything is based on bitcoin prices. If you look at how many bitcoins it takes to purchase a monero let's say, you'll notice the change is within a normal range. This is because they are all too derpy to understand that they should actually be worth more because bitcoins dropped. But they aren't. Why? Because this isn't there true value, it's just FOMO. Sell your alts, get back into btc and stay put.