Why are many insects flying in the air around Tomb-Sweeping Day?

From a scientific point of view, it is not surprising that there are many insects in this season of spring. From another point of view, these insects are reincarnations of the dead, but I believe the former and the latter are somewhat illusory because they have no basis. So I think the reason for your situation is very simple. When you were in Tomb-Sweeping Day, you paid close attention to things flying in the sky, but you didn't pay attention before and after Tomb-Sweeping Day.