Pretty much all coins vary in value like stocks do. There is finite supply of coins based on mining, which is using processor power to solve pieces of the block chain which gets more difficult as it progresses. However no matter the amount of coins that are availible if they are not wanted then the price goes down.
Often times certain events influance the demand for the currancy. The reason it went down this time was due to a Coinbase announcement that they where not going to be allowing BCH trades for a specific period after the hard fork that happened just recently. A big BTC holder sokd off a shiload of coin which started it to drop and of course others started following suit and sold off their coin as well. There are also things like market corrections, people pulling out to invest in stock or alt coins and a bunch of other random shit. Some of which is predictable and others very much not so.
Hope that kinda sumarizes things.