Two caster actions produce a tailing loop: improper (uneven) force application, and a casting arc that is too narrow for the bend in the rod.

• Improper (uneven) force application. The guide calls this one of the two most common causes. Accelerate the rod slowly early in the stroke and much faster later to catch up, and the rod tip dips and rises at some point in the stroke, producing a concave rod tip path.
• A casting arc too narrow for the rod bend. The energy needed for the cast is applied over too short a distance, so the rod tip path goes concave. The fix is widening the casting arc — reducing energy instead leaves too little rod and line speed to make the cast.

Both are concave rod tip path faults. For a straight tip path, force has to be applied smoothly, with the rod accelerating at about the same rate from the beginning of the stroke to the end. Creep can contribute: drag during the pause that narrows your arc leaves you applying power over too short a distance. Practise by reading the fly leg after every cast.

Source

From the CI Study Guide (2025) (PDF), Task 3 and the Five Essentials, plus the tailing loop task; terms from the FFI Fly Casting Definitions.