Why did they switch?

The bill did not pass on September 29 because, despite significant pressure from the financial sector, the real side of the economy had not suffered commensurately with the financial side. That is, neither unemployment nor foreclosures were “high enough” in a sufficient number of congressional districts. The regressions show that legislators who switched their vote between September 29 and October 3 likely caved in to the lobbying efforts of the American Bankers Association. Hence, in all probability the October 3 vote would have failed again had it not been for the pressure exerted by the ABA.

That's from Carlos Ramirez, of GMU and the FDIC. He also taught me macro.