A surprizing number of carapaces from molting Dungeness Crabs turned up in the strand line on Taylor Beach this past week.  Synchronicity of molting may provide an advantage here are a few speculative ideas:

1. There may be a release of gametes from other invertebrates that would provide  a ready food source,

2. It may be to overwhelm predators so that a few crabs survive when they are vulnerable in their soft shell phase , before exoskeleton hardening.

3. Availability of many individuals of a population at once to predators may increase the survivability of those that manage to escape being caught.

4. Its just timing.. they all may have come from the same age egg batch and given constant conditions, have all matured at the same time.