According to Lefty's DVD on flycasting, he refers to a couple of other incorrect tehcniques that affects the loop size:
1. Raising the elbow above the imaginary shelf will open the loop.
2. Rotating the wrist will open the loop.
The number one reason for an open loop is his third principle, a long speed up before the stop.
While he doesn't state it, IMHO a double haul not in concert with the speed up and stop would also open the loop. My speed up is a little wrist with a stop. If my double haul is not during that part of the cast, my loop definitely opens up.