I read on Reddit that big traders (sometimes called whales) buy a lot of altcoins and then there will be an automatic momentum where the smaller traders will cause an upswing of the price, and when the price has reached a certain level the whales sell all their altcoins and buy bitcoins, and the altcoin price drops. The whales keep their wealth in bitcoins and only use altcoins for temporary stunts like that. In this way the altcoin price is being drained.
That is true but normal, its not applicable in all alt coins. Quality coin with good development team like ReddCoin survives.
Big fishes also spread bad news through medias, forum, artificial chart formations to Markdown as in my above post.