After major consecutive rain events, the upper trail surface can hold enough below-surface moisture to allow for effective re-compaction without additional material being added to the surface. There is a relatively short window between “Too wet” and “too dry”. The signs are: (1) about 10-20mm of compaction under foot; and (2) no stick on the surface.
On the right day, it’s possible to convert areas which are usually dusty and rough into a compact smooth surface.