What, you assumed that she'd already supported gay marriage? That she presumably made it clear a long time ago? Nope! Over the past five years or so, as the Democratic Party's most prominent figures switched from the civil union camp to the gay marriage camp — Al Gore in 2008; Bill Clinton in 2009; John Kerry in 2011; Jimmy Carter, Joe Biden, Barack Obama, and Harry Reid in 2012 — Clinton was a notable exception. Even though she backed New York's gay marriage law in 2011 and declared last year that "gay rights are human rights," she hadn't made her support for marriage equality explicit until this morning, in a video released by the Human Rights Campaign.
Not that there's any reason to doubt that Clinton's conversion is genuine, but, as required in any story about anything Hillary Clinton says or does, we must also note the 2016 angle: A Democrat who doesn't support gay marriage will never again be a viable candidate for president. If Clinton intends to run for president, this announcement was inevitable at some point in the next two years, and better to get it out of the way now than right before she starts running, at which point it would just appear totally craven and self-serving.