From what I can tell from looking at some video, my right elbow lifts up at the top of backswing which forces the club up on a steep plane and across the line. I also straighten the right leg a little. From there, it appears that my club gets outside/over the plane on the downswing.
For the past few years, I have fought hitting the ball a little heavy, especially with the longer clubs. I believe this across the line position is causing me some problems with ball striking as the club appears to be coming down very steep into impact.