Broadcast spawners generate high numbers of gametes that are released once an individual is sexually mature. When broadcast spawning occurs, it is often critically timed when conditions are ideal to increase the survival rate of potential offspring. High fecundity increases the chances for large numbers of larvae and overall recruitment for the population. You can imagine how many eggs need to be fertilized if any offspring are going to survive during planktonic developmental stages as many animals feed on that scale.
Based on the generality of understanding broadcast spawners as a whole, one may assume that when the individual organism is ripe with gametes, than almost any physical or chemical disturbance may induce spawning. Any number of factors may be involved, but few that have been proposed; state of tide, lunar phases, temperature change, mechanical stimulation by water flow, waterborne chemicals (pheromones), rainfall. One factor may be the physical bumping of phytoplankton in the water, signalling the availability of food for the soon-to-be larvae.