The reasons are: Newt is popular with Cain's voters (he's the second choice of a plurality of Cain supporters) and Perry is simply not very popular with Republican voters right now, with more Republicans viewing it unfavorably than favorably in several polls.
So updating my thought experiment from yesterday, I will take a shot at seeing what the polls might look like if Cain falls off. In the North Carolina poll from today, 46% of Cain's supporters say they are firmly with him. If the bottom really falls out of his campaign, even those people would leave, but let's just give him the benefit of the doubt. That means 54% -- 16.2% -- would leave him. Newt's the second choice of 29% of those, which means he would see a boost of about 4.7%. So his new total would be 26.7%.
Doing the same for the rest of the field in North Carolina:
Newt: 26.7%That doesn't factor in the undecideds -- which figure to break for Newt as well, based on his favorability numbers and other positives for Newt.
In Maine, only 25% of Cain's supporters are strongly committed to him. Dividing up Cain's supporters gives these results:
Romney: 27.7%Doing static analysis like this is not perfect, of course, but it gives a decent look at the field might look.