The Dutch demise was primarily the same as their rise - economic. Amsterdam was a world-port, being the main warehouse (point B) of goods moving from A to C. They essentially invented large-scale speculation and investment in trade, which was more or less international banking. The decline of Flanders opened up big opportunities for the Dutch, as did their eventual beating of the Hanseatic League.
Jack Merchant points out quite a few important factors in their decline. One important factor was internicine warfare - conflict with the Utrecht eliminated the safety of inland trade. Neighbors also began taking larger portions of the economic pie, leaving the Dutch with less and less. The book 'Kleine Geschiedenis van Amsterdam' (short history of A'dam, if it's in english) explains a lot about the growth and decline of the whole economic climate from well before to well after the Dutch golden century.