Traders read the headlines. They know how the price “should” react to news, and they begin buying. For a while, the prophecy fulfills itself. But then what happens next? It may take an hour or a month, but sooner or later some of the new buyers begin to sell.
Speculators can drive the price quite far in either direction, in the short term. But it is the hoarders and arbitrageurs who drive the price in the long term.