Hornsfan, for me it was a combination of a hip slide and a naturally steep swing causing a negative angle of attack. The solution I implemented was to use a 14° driver until I could sort out my swing.

I'm now back to a higher launching 11° driver. I really have to feel like I'm 'hanging back' to hit it on the upswing. I haven't been to the range to work on it enough... so the 14 rides in the trunk, ready to go at a moments' notice!