I live in an area that usually gets plenty of rainfall. After several drought-stricken years, It has been raining for most of the past month.
It is amazing how many worms I can find now. Anything that has been laying flat on the ground has worms under it. Cardboard, plywood, anything really has worms below now. Some are earthworms and some are red wigglers. Worms even appear in the dog water bucket on the deck 6' in the air!
The chickens love it when I hunt around. They have become picky lately and leave the earthworms be, preferring wigglers.
I have 3 plastic 10 gallon planter- pots full of manure 'worm food'. they have sat on the ground near the garden during the rains. They are full of red wigglers!
This morning I pulled back the tarp that has been covering my carpet bed. Many worms were on top of the carpet trying to cling to the tarp.

I have always wondered why, thinking it was a response to prevent drowning. Several other theories exist, such as a worm trying to relocate while it can, or rain drop vibrations resembling a predator.

I found this article interesting.

Good luck and happy wormin'

Pete

Here is a link that might be useful: rain and worms