Soft-Shelled Eggs

I went out to check on the chickens this morning and there was a soft-shelled egg in one of the laying boxes.  Soft-shelled crabs I'd heard of but eggs was a new one. A quick search online indicated that it was due to one of three reasons:

1. The hen got freaked out by something and dropped its egg before it was ready (possible explanation given some gunfire going on near the shed yesterday - and no it wasn't mine)

2. Calcium deficiency (unlikely but possible, I need to find some oyster shell to make available to them)

3. Hen with an immature shell gland (possible but unlikely given it was an isolated incident).

To other budding chicken kingpins, here's a decent resource on egg troubleshooting.