Because George R.R. Martin has it up on his website. As pointed out by Topless Robot, the excerpt contains spoilers for the end of the previous ASoIF novel, A Dance of Dragons, so fair warning. I'm surprised that he's started on The Winds of Winter, seeing as how it took him years just to finish the last ones. Maybe Game of Thrones reinvigorated him, or perhaps he just decided to finish the series before winter comes.