Carson has been catching Trump, it was only a matter of time until there was a poll showing Carson ahead (apart from the idiotic IDB/TIPP poll that was clearly wrong). It needs further verification from other polls to be certain that Carson is really in front rather than this poll favoring Carson.
If Carson fails in the next debate like he failed in the last debate then the whole dynamic might change, at least in the short term, again.
Carson left a sponge in a patients' brain once! He's not intelligent enough to perform well in debates. Donald will destroy him on stage. Carson's fictional lead will vanish. Trumpster out.