From what I very vaguely remember, the idea is that they're (somewhat accidentally, just by instinct) causing a reverberation in the ground that seems, to worms, like rain. Making the worms (somewhat accidentally, just by instinct) go "aww shit, this lovely dirt hangout spot is boutta turn into a bathtub, I gotta get outta here!"

It's pretty cute watching em on actual dirt. Especially with a young'n, the little ones figure it out pretty quick. Somewhat accidentally, just (largely) by instinct!