Seriously, I really don't see why he's falling. It's kind of sudden. Expected, but I hadn't really see the signs of it happening.
Everyone remembered Newt Gingrich was in fact Newt Gingrich.
Unlike previous FoM, I don't think there's a single identifiable cause. Newt has been under attack on all flanks for two weeks now, and while nothing in particular has brought him down, many of the accusations are beginning to stick. If I had to guess what the most important factors behind Newt's decline are, I'd blame:
1. Newt's time as an influence peddler
2. Newt's comments on Romney's tenure at Bain capital
3. Boredom
4. Voters are starting to take a closer look at the race, including many who were shallowly supporting Gingrich based on name recognition or vague familiarity with his record.
I think that a lot of people are too quickly assuming that Gingrich will, like his predecessors, quickly and drastically bottom out. I don't necessarily expect that to happen before the caucuses. Barring a major incident, I think that Gingrich will steadily bleed support in no particular direction. If there really is a dedicated anyone-but-Romney portion of the electorate, it won't be until after the early contests that anyone is able to consolidate their support.
