Remember that fly line always follows the direction of the rod tip, so a concave movement pulls the upper leg of your loop downward into a tailing loop. This concave movement can also be caused by lowering your hand as you begin your cast, then lifting during your power stroke. A friend of mine throws beautiful tailing loops. He uses a short stroke and applies power with a strong wrist snap. Too short of a casting stroke is being made to cast a long line, causing the tailing loop.
Beginners have a particular problem with unequal power.
They will lift the rod weakly in an underpowered back cast, then hammer or overpower the fly rod to the front to try to lay out line. The top leg of the loop drops to hook over the bottom almost every time. Lefty Kreh has another explanation for tailing loops made by advanced casters. Casting tight loops by driving your rod in a straight-line path can also cause your trailing line to crash into the forward part of your loop. This happens with an extremely stiff wrist that locks the rod on one track.