They wanna be with their mates, their metabolism sky-rockets, they metamorphosize, swarm and go like HIAG in a pie shop. Too many physical encounters with other locusts causes it, so when the population increases to a certain point the gregarious phase is a certainty. the largest known swarm covered an estimated 198,000 square miles with between 3.5 and 12 trillion locusts https://en.wikipedia.org/wiki/Albert's_swarm
