That is called a tailing loop. It is when the fly leg of the loop falls below the rod leg of the loop. The as the loop unfurls, the fly leg tangles on the rod leg. It causes "wind knots."
I suspect that as you try to shoot, you add a bit of extra "oomph" to your forward stroke too early in the casting stroke. That causes the rod to bend suddenly. Because the rod bends, the fly rod tip dips down causing the line to dip down. Then the rod elongates at the stop carrying the rod leg of the loop above the fly leg ====> tailing loop.
You need to smooth out your final forward casting motion. A tailing loop is the third cast in the illustration below.
Here's a video of a tailing loop.
Besides smoothing out the casting stroke, there are two other things you can do. You can tip the rod down with what is called a "micro wrist". This micro wrist controls loop size.
A wind knot occurs because the following fly leg (upper leg) of the casting loop falls below the standing rod leg (lower leg) AND the legs are in the same casting plane. BOTH situations must occur, that is the following leg must cross the standing leg and the legs must be in the same plane. A wind knot cannot occur if the two legs of fly line are in different casting planes.
Example - By using an elliptical casting motion, the back cast and forward cast are made in different planes and this separates the two legs of the loop formation. Even if the fly and rod legs of the loop formation cross vertically, they cannot catch on each other because they are separated horizontally in space; they are in different planes.
To see how this works, make a side arm back cast and then an overhead forward cast and you will see than the two legs of the loop are in different planes. Even if the upper fly leg of the loop drops down on the forward stroke, there is no lower rod leg of the loop to get tangle with because there is a horizontal separation of the two legs of the loop.
This type of cast is known as the Belgian Cast. Because this cast separates the planes of the back cast and forward cast, it is an excellent cast to prevent tangles not only for tailing loops but also when casting multiple flies or heavily weighted flies. It is also an excellent wind cast when your back is to the wind and it often called the Belgian Wind Cast for this reason.
However, the elliptical motion also causes the fly line to twist counter clockwise for a right handed caster. By casting in an ellipse we are moving the rod tip in a circle for each cast and this twists the fly line. If you always use the elliptical cast, you'll need to allow the line to untwist every so often.
To remove the twist, make a long cast downstream, and then opposite
clockwise loops with your fly line to remove the twists and then reel up the line onto the reel before leaving the stream.
Here's a video of the elliptical cast. You can see how he would introduce a twist with his casting motion.
"The disadvantage of this cast is that it throws a half twist in the line every cast. Half twists add up! So it's best to use this cast sparingly, otherwise you will have to get into the habit of removing the reel from the rod, every 30 casts, or so, in order to spin the twists out."
The Belgian Cast