I am completely speculating here, but I think it is a “numbers” thing… they emerge in such high numbers that the larvae rapidly consume available food supplies, and not finding a large amount of food plant available for oviposition the adults tend to travel in a certain direction looking for more larval food plants, perhaps following the direction in the sky that the sun generally sits, so tending south in your winter and north in your summer. Here in NZ we don’t get a large amount of them, maybe a tree overwintering a few hundred, but certainly not the large numbers in one place like is seen in North America, so larval food plants recover at a rate that can keep them happy…