Cain’s ascendency began after his win in the Florida Straw Poll which was probably a month before Palin announced that she was not running but if it makes you feel better to think these things by all means continue.
Florida Straw Poll was on September 25th. Palin’s announcement was on October 5th.
Pretty clear from the RCP polls that Perry’s fall started with Florida but Cain didn’t overtake Romney until after Palin’s announcement.