Mating.
Rain cause eathworms to come to the surface (food) and creates a cool and moist enviroment, these are perfect conditions for Frogs whihc they often use for breeding. And since Frogs often return to the same bodies of water in which they were born, you see this purspseful mass migrartion with our roads in the way.
Btw Frogs can sense the upcoming rain (humidity in the air) and will start to become louder just before it (to attract mates)
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