My Mom has raised red wigglers and one thing that I encountered in having my own bin, was that if the material inside was too wet and/or there was water/moisture gathering at the bottom of the container, the worms would surface in order to avoid the standing water.

Have you drilled holes in the bottom of your bin? (large enough for drainage but small enough that the worms can't runaway)...doing this and setting the bin up on blocks so that it can drain properly is helpful...and if your bin is indoors then you can set a catchpan under the container so that the excess moisture doesn't run all over the place.

Hope this is helpful...