The price of each coin is determined by how much are people currently willing to spend for it. To simplify lets just say that the price of the coin is determined by the price of the last buy or sell order.
Lately due to bad publicity and other factors cryptocurrencies aren't receiving much love so not many people are willing to buy but more people are willing to sell so in order to sell they need to sell for a lower price. And because bitcoin is like a bridge to other alt coins, if less people are buying bitcoin its expected that less people are also buying everything else and so everything is dropping.