How's the weather? When it rains (or the weather changes) and the Barometer drops, the worms will start heading for the surface.
I'll let others fill in all the other possibilities.

Shaul

Keep the light on in your bedroom?

The last few days 1/5 of Americans were exposed to serious weather. My guess is you might want to provide more surface area and bedding to the worms. "More bin please?" Possibly search out more food source. Can you sell or gift some of the worms. Please keep posting because obviously you are doing something very right raising worms. Have you ever harvested vermicastings? What is the reason for vermicomposting? Did you meet your goal?

Thanks equinox! Your comment about bin size is I think accurate as my worm population has grown exponentially. But shouldn't some of them just die off then to achieve a more sustainable population? Weather is not a factor as my bin is in my basement. I have harvested three times in the past year, but that's another problem. The bedding (shredded newspaper and cardboard) is not decomposing as rapidly as the worms are creating castings so I end up with the bin chock full of castings with cardboard and clumps of wet newspaper embedded throughout. I use a screening box to separate the castings from the paper but I would prefer figuring out how to get the generation of castings in sync with decomposition of paper. Any ideas?

Screen dryer castings. They are easier to work with.
Let the result age with the few inevitable worms left in it.
Decide vermicompost fits your needs almost as well as pure vermicastigs.

I thought pure castings was what everyone achieved when they used worms. But you're implying a difference between vermicomposting (castings with some "stuff" in it) and pure castings. Looks like everyone deals with the same problem. I don't mind screening vermicompost to get pure castings.

And the stuff screened out is great for inoculating new additions or a new bin.

Great. Will try it. Thanks for your help.

I wish my problems had been the one you have instead of some I have experienced. Population explosion causing crowding.

wormwarden: what do you feed them?
Condensation will cause worms to climb up the walls and lid.
Your worms might be looking for greener pastures where there's more chow, lol.
Smaller grocery stores usually won't mind to let you have their discarded produce.

If when you close down the bin cover, the worms start climbing the walls, then maybe they're afraid of the dark. :-))

Shaul