As a rule of thumb, any curvy shape in the fly leg of the loop is the telltale of some kind of casting problem. The most popular (or should I say unpopular?) of them being the tailing loop. But not all of those shapes are tailing loops. Another usual curvy shape is the dangling end; it is pretty recurrent in long casts —although, depending on the conditions, it can appear in short ones as well. Clarifying the difference between the two is not a trivial issue, as each one has a different origin and —more important— a different cure.
A tailing loop is a transverse wave in the fly leg of the loop whose origin is a subtle swinging (down-up) of the rod in the same plane of the rod. That implies that the resulting wave starts at the rod tip and propagates along the line as the cast progresses.
On the other hand, a dangling end is a wave in the fly leg of the loop whose origin is a misalignment between the rod tip path and the line position before the start of a casting stroke. In this regard, the curved delivery that sometimes results from a tracking problem is a case of dangling end. The main difference between a dangling end and a tailing loop is that the wave gets formed far away from the rod tip.
The following video explains the process of the formation of a dangling end —by the way, it is amazing how the quality of slo-mo has improved over the latest years.
For more insights on tailing loops: