Most of all bitcoins are concentrated on a small number of wallets. The one who owns so many bitcoins can very easily raise or lower the price. The price of bitcoin greatly affects the prices of altcoins. So I think that the price is manipulated by big players.
But that is only natural, the one that controls the most coins can manipulate the market, many people think this as something unfair but that is not the case because any person with so much power can manipulate the market even if that is not his intention, a person that is only holding his coins so he can sell later is manipulating the market by reducing the supply even if that is not his intention.