When available I've always added crushed egg shell. Composting can bring about acidification so the egg shell would help stop pH from getting too low. Some people add lime for this reason. I've also added wood ash for the same purpose. It has a much higher pH and worms move away from that until it's reacted a bit. I'd agree with Embothrium, probably some other factor coinciding with your addition of the egg shell. Try digging down to see if the worms are lower in the can.

I sprinkle my egg shells ontop of the dirt to control snails and slugs.

Hubs just throws al our egg shells in the compost. Doesn't bother breaking them. When he turns the piles over they get broken up in small pieces. He's been doubg this for 40 years.

I just crush them up by hand and throw them in my gardens some times in the compost. I don't know of any effect it would have on worms sorry can't help there.

Perhaps all that powdered eggshell made the worms go looking elsewhere for food? They eat dead plant matter, not ground up minerals, after all.

Alternatively, their disappearance may have had nothing to do w/the eggshells at all. Correlation does not equal causation.

I also just crush the shells by hand and have tons of earthworms. We go through at least 2 dozen eggs per week too.