Why did the children tumble upon each other and run?

We learn in "All Summer in a Day," that after a brief hour of sunshine, the rain on Venus is starting to fall again:

A boom of thunder startled them and like leaves before a new hurricane, they tumbled upon each other and ran. Lightning struck ten miles away, five miles away, a mile, a half mile.

The children run and tumble to get underground because the rain has returned.
Before this, they enjoyed the glorious time they had in the sunshine. None of them remember the sun, as it is only sunny once every seven years on Venus, and the children are merely nine years old. They run in circles, thrilled by the new experience. Then one girl caught a drop of rain in her palm and began to cry. The children become sober as the rain pours down—and then they suddenly remember Margot, who has been locked up for the entire period of sunshine.