i posted a similar question about this today. this is a very old post that doesnt have any closure, so i was hoping those that chimed in are still around. i'm currently trying to figure out the exact issue.
one thing i noticed was his impact came into question a lot. i'm not sure if he had the same issue, but i can swing at nothing and have the same closed face result. So it's not the impact cuasing the turn for me. in fact, i know that my issue will happen prior to actually doing ANYTHING. it's happened to me enough to realize when it's going to happen, before it does (the moment i address the ball). so i know it HAS to be my grip.
more specifically, i think i'm griping too tight. i just started golfing late last summer and played all winter long, even got my first eagle in 32 degree weather from 152 out. i think that as i begin to sweat more, i'm subconsciously gripping tighter as the thought crosses my mind from time to time that i'm going to lose my club in the backswing. however, i just stop my swing and start over and usually am fine. the real issue is when i know it's coming before it happens and at that point i'm done for the day. this just started during the hotter months.
anyways, 2 nights ago i finally broke down and checked my grip pressure. i was really sweaty and it started happening, and was bad. i took my dripping glove off and wrapped a towel around both hands and the club. problem solved!! i actually hit both my 7i and LW 20 yards further than ever. .with total control and complete "feel" during the swing. unfortunately, it was at sunset and i havent had a chance to test beyond those 5 shots, so i'm stoked about my range session i'm hitting today.