I think a lot of the earlier polls were drastically over-sampling Democrats. It's not that Romney picked up more likely voters, but rather the polls were skewed the wrong way to begin with. I think the pollsters realized their credibility was on the line as people started digging into their methodology and the election got nearer, and finally corrected. After all, if they were still polling +9 or +11 D and showed an easy Obama win and Obama gets stomped, nobody would take them seriously again.
Edit: As for why, I think it was a combination of DNC-run polls trying to make their guy look better and the others (whose employees probably skew Democrat personally but weren't purposely trying to bias their research) basing their assumption on a flawed Democratic turnout model. They thought 2012 is going to be like 2008, and ignored the 2010 midterms and other evidence that the Democratic base simply isn't very enthusiastic.