Wind knots caused by tailing loops are common when making longer casts with extra effort. The can occur on both the b ack and forward cast. Almost all tailing loops occur when the rod tip speeds up and stops in a straight line.
Here's why. When casting a fly, the line unrolls in a loop. As long as the top of the loop remains on the top and bottom on the bottom, there will be no tailing loop. To avoid almost all tailing loops, the tip of the rod must get out of the way of the incoming line to avoid running into itself. As you come forward, dip the rod tip slightly on the stop. More than a slight dip will open the loop.
The above is compliments of our esteemed sage, Lefty Kreh.
One additional point from Lefty: "If your rod hand travels at the same height or plane throughout the forward cast and on the speed-up-and-stop the thumb is parallel to the water (vertical or side casting), no tailing loop occurs."