Here is some information from Wiki regarding the term limits extension:

In the fall of 2008, Bloomberg successfully campaigned for an amendment to New York City's term limits law, in order to allow him to run for a third term in 2009. On September 30, 2008, reports emerged that Bloomberg was seeking to amend the law,[5] and on October 2, 2008, he announced plans to request the removal or extension of term limits for elected officials. On October 23, 2008, the New York City Council voted in favor of extending the term limit for elected officials to three consecutive four-year terms, thus allowing both Bloomberg and other council members to run for office again.[6] During the amendment campaign, Bloomberg's administration requested the support of certain nonprofit groups, which attracted some criticism because these groups have received threats from the mayor in the past..

I am not alone in the opinion that Mayor Bloomberg used his financial influence to extend the terms limits. Despite how unpopular this move was, I'd have to say I'm not surprised that he won.