"the lock was a falling, and not a spring, one"
by cm
Lock Mechanism
Creative Commons Attribution Share AlikeLock Mechanism - Credit: Mike Gifford

A spring lock has a spring-loaded bolt which shoots automatically into place on closing a door.   Accidental entombment would be a risk with such a lock, given that tombs generally do not require keyholes on the internal side of the door.

We can't find any description of a "falling lock", but presumably  – like a deadbolt – it cannot be locked without a key.  Further information would be very welcome: editor@bookdrum.com.