The Honeycomb grouper, one of the smaller fish species in the Epinephelus genus, sometimes undergoes spontaneous sex changes. ref. Wikipedia

The honeycomb grouper is a protogynous hermaphrodite. A rapid drop in endogenous E2 levels is believed to trigger onset of sex change in protogynous wrasse, Thalassoma duperrey (Nakamura et al. 1989). However, the physiological events that trigger female to male sex change in groupers are unknown.


Source : Fish Physiology and Biochemistry 28, 141–142

The study cited above found that the changes are strongly associated with hormonal changes, and can be artificially induced. But what triggers the changes in the fish's natural environment, or why the trait has developed through evolutionary time, remain unclear,