I've seen before where some of the worms from my inside bin escaped and dried out on the floor before and during a heavy rainstorm. Maybe they can sense the front moving in like other wildlife; and feel like they have to evacuate to avoid drowning?

i think that was the case- i've seen no dried up worms since this last post. bin smells great and no escapees!! thanks for the feedback, eric30!

I have seen worms crawl when the barometric pressure is low, but I think this is something different.

If you have a standard Rubbermaid bin and you use a dump and sort method to harvest, then you are basically starting the whole process over every time you harvest. Just like worms tend to crawl for the first few days when you buy them, I suspect they do the same thing when you harvest, then put them back into a bin with all new bedding.