When the Maine legislature first legalized it in 2009, and then a counter-campaign put it on the ballot, and Maine voted to unlegalize it, Susan Collins didn't do favor it.
When Mainers United for Marriage organized in 2011 and 2012 to put it back on the ballot, Susan Collins didn't support them.
When their referendum passed, Maine legalized same sex marriage again at the end of 2012, and in the year and a half that it's been a reality in Maine, Susan Collins didn't support it.
Why now? Because earlier this June, Shenna Bellows became the Democratic nominee for Senate in Maine, running against Susan Collins. As executive director of the Maine ACLU, Shenna Bellows was one of the lead organizers of Mainers United for Marriage. She was one of the people who formed the organization, back when Maine had recently voted to ban same sex marriage and many people said it was too soon to try again. Her active leadership for marriage equality before it was won has been earning her campaign a lot of support, and contributions.
So what did the HRC do? They struck a deal with Susan Collins: She says she favors gay marriage, and they endorse her for Senate over Shenna Bellows.
I guess the message to other politicians is: You don't earn HRC's support by working hard for equality, which is supposedly their mission. You do it by sitting on the sidelines until the work is done, and then telling HRC that if they'll endorse you, you'll say that you favor the gains from the work that other people did, after they've already won those gains.
Edit: A post by digsby, via jered, that expands on this. While HRC points to Susan Collins' support of the current Employment Non-Discrimination Act in the Senate, she's part of the reason why the current ENDA is so relatively narrow in its protections; Shenna Bellows, on the other hand, led in getting Maine to pass the Maine Human Rights Act, with broad and comprehensive antidiscrimination protections.