The behaviour is called an arribada, and only the two ridley species do it. At Ostional in Costa Rica and along the Odisha coast in India, arribadas can bring several hundred thousand females ashore in a single event, with each digging a chamber in sand already full of eggs.
The cost is severe and the benefit is statistical. Dug-up and rotting clutches lower hatch rates within an arribada, but the sheer simultaneous output swamps the capacity of vultures, coatis, crabs and other predators, so a higher share of the hatchlings that do emerge survive their first minutes.
What triggers a given arribada remains unresolved. Lunar phase, offshore wind, and social cues among waiting females have all been proposed, and none predicts the timing well enough to schedule field work around it.