The price and, ultimately, the charts on any given exchange are determined by the price of the most recent transaction. How can the prices of 100s of cryptos all move in the same direction at the same time?
This is the trend, if the top coins goes down the thinking of the trader/investor is to save their money from the crash and that is the time for them to sell according to the market trend. This is pretty normal especially if the altcoins is paired to BTC since this is the main basis of the market, if BTC dump many will follow later on. Though some are still able to go for a different trend, but you rarely see this kind of trend most of the time, with cryptomarket BTC is the leader and the initiator of the market trend.