Looking at the loops in fly casting we define every loop to have a fly-leg (fly to loop front), a loop front (transition) and a rod-leg (loop front to rod tip). A tailing loop to me is a wave in the fly-leg. This wave can be small, medium or sometimes even big. Often it will be a small wave slightly growing in it's size during unrolling and then ruin a smooth final turn over.
The physicists among us fly casters may prefer to call it a transverse wave propagating down the fly-leg (directed tip to fly).
In order to bring the fly efficiently to the fish we must match line speed, loop shape and trajectory.
For that we rotate the fly rod to create the desired line speed best matching our fishing situation. As a consequence the rod bends. In order to shape proper loops at the same time, it is very important to keep the increase in rod bend smooth enough. Any sudden increase in rod bend instead and we draw a wave into the accelerational rod tip path resulting in exactly that transverse wave propagating down the fly-leg of the loop.
Looking back into my teaching experience, the causes for a too sudden increase in rod bend were:
1) A too uneven force application for the rotation in order to compensate previous issues like
a) early rotation during unrolling not leaving a proper size of arc left for the next cast,
b) too open loops making it hard to wait for the line to unroll,
c) missmatching line speed (typically too high) or
d) too much slack line.
2) A floppy/incontrolled wrist in general leading into a too uneven force application.
3) A too uneven force application by the hauling hand.
4) A too bad timing of the hauling hand movements (typically finishing the hauling hand movement too soon).
Bad timing in general and not having the urolled line matching trajectory for the next cast all can make it worse!
The most important key factor?
As long as you keep the rotation (and as a consequence the increase in rod bend) smooth, it get's very hard to shape any tailing loops!
I wish you all a perfect first cast and a strong pull at the end of your line. I've just done it a few times myself this week. ;)
All my best
Our last days...