[Petrochemicals (OP)]

Is it weather, luck, food avaliability ?

[alancalverd]

They don't know about food availability when they set off on their travels, but it is quite likely that weather history has a lot to do with it as the swarm  depends on simultaneous hatching and maturity of an entire population. Some species have evolved a corporate life cycle that does not synchronise with their predators, so the locust maxima occur when predator numbers are minimal and there are never enough locusts around the rest of the time to sustain a large breeding population of predators. 

[sciencefreak24]

In the wild, swarms usually appear after a rainy period followed by a time of drought. After rains, populations of grasshoppers explode, because there is food aplenty. But when the land becomes parched and grass scarce, the populations get pushed into smaller and smaller areas, becoming more packed as desirable pasture diminishes. At a certain point of density, the swarm-inducing serotonin gets triggered and the locusts set off en masse to find greener pastures. After that, few things — other than an end to the food supply or an ocean — can stop them.