I started reading the A Song of Ice and Fire series almost 15 years ago and have gotten used to waiting years between books - I haven't watched the show for the past two seasons and am not too thrilled he is rushing due to HBO's series catching-up:
Of course, that isn’t the only reason Martin might be in a hurry.
Considering that the sixth season will most likely premiere before ‘The Winds of Winter’ are released and events even in the fifth season might already contain new material it would be no surprise that Martin wants to try to get his next novel out before the show spoils it for readers. It will be an odd twist of fate with how many readers have spoiled the show for viewers so far.
The showrunners do seem as if they are probably ending the series after the seventh season which may or may not spoil how the novels end depending on changes that are made and how quick Martin is able to finish up his series. We already know that the next novel won’t be released in 2015 so he’ll have to be in a hurry if he wants to try and beat the 2016 sixth season premiere.
http://sciencefiction.com/2015/06/02/george-r-r-martin-busy-working-winds-winter-write-game-thrones-season-six-episode/