I would argue that Snowe's independent streak mostly stopped in the last two years. The
New York Times notes:
http://www.nytimes.com/2012/02/26/us/politics/republicans-stampede-to-the-right-ahead-of-2012-election.html?_r=1&pagewanted=allShe's been catering to the Tea Party of late and her statement suggests that she didn't like it. It's worth noting that this is not a Ben Nelson style exit where Snowe is quitting because she probably would have been defeated. She's leaving because she no longer wants to be in the Senate.