Opened out to show what was folded away. The contents were always there; the state changed.
Latin expandere, ex- (out) plus pandere (to spread out); the verb moved from the literal spreading of a cloth or a wing to the general opening-out of anything that had been folded away, including a state waiting to be shown in full.
Opened out to show what had been folded away. Nothing new arrives when a thing expands; only the state of what was already there changes.