Randy did a nice job going through a few potentials. You might also consider that you are stopping the rod too upright on the backcast or perhaps simply not loading the rod up enough on the backcast and the popper is simply falling. While you don't want too much wrist action, you do need some, so let the bottom of the flyrod develop a 45 to 50 degree angle between your wrist and rod butt on the end of the backcast. This will move the tip just past upright. Also, make sure that you give yourself what Joan Wulf calls a strong loading move on the backcast. This is a slight upward motion with your casting arm and shoulder. As you complete the loading move and break the wrist the line should unfurl up and behind you. On your front cast make sure you take advantage of that loading move and bring your shoulder and arm down first before extending the forearm and snapping the wrist at the end (Joan Wulf calls this the power snap).
Hope that makes some sense!
Also, there is a good site that discusses fly casting techniques at Fly Fishing Tips - Fly Casting - MidCurrent
EXCERPT: Casting Large Flies
When casting flies that are heavier or larger than what your line and rod were designed for, try putting a side-arm motion in your back cast and making a straight overhead forward cast. This is sometimes referred to as a "Belgian" cast and can also help when the wind is blowing over the wrong shoulder. Be aware, though, that this style of casting will also put twist in your fly line. (See "The Belgian Cast.")