K3 - been there done that. Jerry is right, it is from overpowering the forward stroke and probably from having the rod tip not directly overhead, but off to the right at an angle. I would always get excited just before the final forward cast, just before releasing the line, and I would try to punch it out ahead. As a result you overpower the forward power stroke. Don't speed up or power up on the final delivery. Just relax a bit and stop the rod at the right height. The rod will unload like a spring and generate the shooting power you need.

The bungee thing sounds like you might be stopping the rod abruptly at a high position.

JC is also right, don't necessarily unlearn these two casting phenominons. The first method is a curve cast, which if you can exploit is a mighty nice technique, and the second is a tuck or parachute cast, which is great for getting nymphs down to the bottom quickly.