History Will Note... Then Wonder... Why?

From Rasmussen:

Unless McCain pulls off a stunning comeback, history will note the final two weeks of September as the decisive and defining moment of this campaign. On September 14, the Rasmussen Reports daily Presidential Tracking Poll showed McCain up by three points. Then, Lehman Brothers collapsed and the Wall Street debacle began. McCain’s lead disappeared almost immediately. By September 26, Obama reached the 50% level of support and was ahead by five percentage points.

Why?