The Occupy Wall Street movement - for now it is a movement - is the most important political happening in the United States since the uprisings in 1968, whose direct descendant or continuation it is.
Why it started in the United States when it did - and not three days, three months, three years earlier or later - we'll never know for sure. The conditions were there: acutely increasing economic pain not only for the truly poverty-stricken but for an ever-growing segment of the working poor (otherwise known as the "middle class"); incredible exaggeration (exploitation, greed) of the wealthiest 1% of the U.S. population ("Wall Street"); the example of angry upsurges around the world (the "Arab spring," the Spanish indignados, the Chilean students, the Wisconsin trade unions, and a long list of others). It doesn't really matter what the spark was that ignited the fire. It started.
In Stage one - the first few days - the movement was a handful of audacious, mostly young, persons who were trying to demonstrate. The press ignored them totally. Then some stupid police captains thought that a bit of brutality would end the demonstrations. They were caught on film and the film went viral on YouTube.