It could be caused by a number of things but I would focus on your setup, especially your alignment and grip.  

I agree with Nick, I have recently corrected a duck hock similar to yours, and it was basically my allignment that was causing the hook! I realised I was aiming right when i thought i was aiming straight and was swinging the club towards the target therefore promoting a draw and was sometimes turning it over even more resulting in a duck hook. Also my feet were aligned wrongly with my left foot considerably infront of my right therefore promoting a duck hook or draw bias yet again! Although there are many other factors that could result in a duck hook Eg - Swing plane, Grip, etc.   

I am working on getting rid of mine. It's due to getting stuck on the inside on the way down and flipping the club head for a release instead of following around. My pro said I am trying to guide the club instead of swinging fast. First time I have ever had anyone say I swing to slow with a 105 mph swing. But now im up to a fast, not hard, 115 mph swing. I complain to him now that i drive the ball 30 yards further (in to a lake), and my hook to the middle of the fairway is now a straight shot towards the right ruff. Once I trust that I hit it straight and club down, watch out!

well lets start with the basics Grip.Alignment,Posture and ball position

Has your grip gotten a little strong? check it against pictures in any golf magizine

are you lined up correctly?? Use a club to check your feet and a friend to check your shoulders and hips

Have you let the ball move ahead in your stance? it should be around your instep

and make sure your posture is a neat athletic stance not to straight up nor too bent over

Good Luck

I USED MY ALIGNMENT STICKS AND AFTER GETTING ALIGNED RIGHT THE SWING TOOK CARE OF ITSELF.