That's what happens when your worms develop an attitude and decide there must be better food somewhere else. I checked the worm bin one morning to find a couple dozen worms on the floor.
Now, we know there's a problem, but it's really hard to figure out exactly what. It could be:
All we can do is try to figure out what has changed recently and correct the problem. In my case, I noticed a few days back that the usual condensation on the inside of the bin was gone - indicating a dryer environment than normal. I also had spread a a larger than normal amount of bird pellets left in their cages. And, I had put a handful of damaged sweet basil in the bin. There's three things that could have affected the worms to the point of them wanting to leave the bin.
However, since there was no food in the immediate area to draw out the worms, it likely isn't a too-little/too-much food problem. It could be the moisture problem is drawing the worms down into the bottom of the bedding where, in turn, they fall into the collection area and exit.
The basil could have been the culprit, as it contains some concentrated oils that could repel the worms. I'll have to research that a little more to see if it's a real problem for them. Meanwhile, no herbs into the bin.
Finally, there's the bird pellets. We've put pellets in before with no adverse affect, but not this large an amount at once. The pellets did pull a lot of moisture out of the bedding, forcing the worms to the edge of the bin where the moisture is higher.
We might never know what the specific reason was, but we can certainly say what the cause was; Too many large changes were made at once to understand the affect on the worm ecosystem. Any single change might have been tolerated, or not. However, it is best to stay with what works and be consistent. Test any changes in a smaller bin to make sure it won't cause a wholesale exodus like I experienced.