The tailing loop is due an even haul OR not tipping down your wrist just at the stop.
When the fly rod straightens after the stop, the "effective rod length" (the distance of the rod tip from the hand) lengthens. If you don't tip the rod tip down to compensate for this rod lengthening, the rod tip will be higher than the trailing fly line causing a tailing loop that begins at the rod stop. You need to flick the rod tip down just before the stop to allow the fly line to clear the rod tip.
There are many names for this flick. Doug Swisher calls it the "micro wrist". See this description from the Orvis Guide to Better Fly Casting
by Al Kyte, pp 25/26.
The Orvis Guide to Better Fly Casting
"Most instructors teach a firm wrist throughout the forward cast to reduce 'wristiness,' but they probably hope students will move the wrist enough to help speed up the tip. I am careful to avoid the term locked wrist, because some students do exactly what you say and then have trouble loosening their grip enough to use the wrist at all. And if you even mention wrist movement in your teaching, students will often overdo it. Knowing this, some instructors teach students to press the thumb. In doing so, they teach a little late wrist movement, without having students even think about the wrist.
Longtime Orvis Master Instructor Bill Cairns has taught tightening the thumb and forefinger, bringing the the wrist into play and stopping. Doug Swisher referred to this quick wrist movement as a micro-wrist. and Joe Humphries refers to it as a tap……
Jim Green also emphasized pressing with the thumb to create a little wrist movement before stopping it immediately before his hand. So the "positive" in his positive stop is a little wrist pivot that not only stops the rod, but helps force the tip over the resistance of the butt of the rod."
When the rod tip travels in a straight line, loop size is controlled by a micro flick of the wrist just before the stop. This micro flick speeds up the rod tip and moves it out of the way of the following fly line. It tips the rod tip down a bit and controls the loop size. Otherwise, the "effective rod length" lengthens as the rod straightens at the stop. The rod tip moves above the level of the following line and a tailing loop develops.The size of this flick controls the loop size.
The Illustration below is from Jason's book on casting and shows that the degree of "flick" controls loop size.
Fly casters who think the wrist must be absolutely locked
are surprised when they learn of the late micro flick. But it is a necessary move for a good cast and to prevent a tailing loop.
The Federation of Fly Fishers Master Casting Clinic Study Guide states:
"The wrist is better suited for quick, final movements than for those requiring sustained, evenly applied force. ------- Other instructors believe this wrist action is so important that they emphasize it in their teaching. Lefty and Joan Wulff cast with different styles, yet both have stated that they use large muscles to provide force and direction to throw the line, but a late, quick wrist movement to control the size of the casting loop. ------- Longtime East Coast instructor Bill Cairns has similarly described this wrist action. Doug Swisher has taught it as a "micro-wrist" movement and Joe Humphreys as a "tap". ------ On the forward cast, I want to build in wrist action as part of the stop. To do this, I need to channel a student?s wrist movement into a late time frame within the cast."
I use a wet paint brush as a good way to teach this wrist flick or micro wrist. I ask the student to stand in front of a wall and flick water off of a soaked paint brush so the water lands on the wall at about eye level.