Probably setting the tee too high or the ball too far back in your stance.

This happens to me when I my head and shoulders get really far behind my hands at impact sometimes, or if I tee it too high. 

I had the same problem a while ago and finally figured out what I was doing after lots of trial and error as well as asking my local pro. That being said, Pop ups are caused when you come in too steep and loose your spine angle. If you focus on trying to shallow out your angle of attack and try to keep your spine angle and balance, the pop up will stop eventually. Good luck!

I started doing this a few weeks ago and found help right away. What I was doing was on my down swing I would try to hit the ball farther and bend my knees and go down on the ball which made me hit it high and left. Once I kept upright through the swing and realized that I should not try so hard to hit the ball, the problem was basically gone.

I have noticed some of my drives doing the same thing, but it was due to me teeing the ball up to high.  Might try that out and see if it is the culprit. 

Generally, when you swinging from outside to inside it will cause pop-ups. So increase your reverse pivot, so you can swing from more inside to outside. When you come over the top, the clubhead sometimes will come into impact on a steep angle of approach, causing it. I had the same problem  a while back. The driver is the club you do not want to swing "down on the ball with, but up on the ball"