We can't say why Gallup is moving. But we can say that a dramatic new move for Obama coincides pretty much exactly with the release of the 47% remarks on September 17th.
Gallup is a 7 day running sample. That tied period after the convention bounce starts on the 19th and ends on the 21st, just as the post-17th sample would have begun to build up in the 7 day sample.
Today's number, where Obama ballooned to a 6 point lead, is the first day the entire sample is after September 17th.
Again, no proof of cause and effect. But the correlation is extremely tight.