How do worms get in my pool after a rain storm?

Question

How do worms get in my pool after a rain storm?

Answer 1

They are flushed in there from the wash away.  The same way that water always find a drain.

Answer 2

Worms come up out of the ground when it rains to avoid drowning.  They aren't adapted to being above ground and can't prevent being washed all over the place in a hard rain.