Just understand the coin info by following the general discussion groups to understand which coin is on Hype,after research you can get idea about the  project .Might be people consider to sell the coins when the bitcoin price falls because that the whole crypto market is manipulated by whales.who holds the high amount of coins, they might sell the coins in bulk which causes the huge fall on the prices,my method is when this happen sell and turn usdt to your eth or btc when its down you can collect more coins