Possible causes:
1.The club could be getting too far behind you in the backswing, causing the weight to go with it, slightly off balance weight too much in the heels resulting in a compensation in the transistion which sees the weight move towards the toes of both feet and hence the loss of balance in the downswing.
2. Backswing too upright, right elbow too high, clubhead and hands move out towards the target line in the transistion, weight goes with it.
3. Poor transistion move, "roundhousing" in TGM speak, right hip and shoulder moving out towards the target line instead of moving towards the target, result too much weight in the toes. Try sliding the hip parallel to the target line, not towards it. The weight should be moving towards the base of the toes of the left foot.
4. In the transistion the shaft fails to flatten, clubhead and hands almost lined up, right elbow and right shoulder high, steep start to downswing, weight towards target line.
Hope this helps!