Suddenly hundreds of earthworms have left my lawn and crawled on the sidewalk and alley to die. What caused this?

People used to think that water saturated soil was the cause. The current thinking has to do with lightning & thunder causing the worms to commit suicide. Sounds goofy; might be natures way to thin the worm population.

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The "soap" you use is normally chemicals, etc. Use real SOAP !!

Often people think that the reason worms crawl out on the sidewalk when it rains is to avoid drowning when their holes fill with water. A few months ago, after the author of an article in Discover magazine made a similar claim, a scientist who studied worms wrote in to say worms can live underwater as well as in the dirt. They don't breathe as we do but get oxygen in some way that makes drowning an impossibility. He said the reason they come out was mating.
I remember from Biology class that worms are hermaphroditic, having both male and female organs, which might disprove the mating theory. But I also remember seeing a video in class showing how they mate and have observed this myself. So, I believe the mating assertion is still correct.
Tony M