The term "Retrospective Astonishment" (perhaps someone could translate this into Latin?) does capture the point. I'll endorse your term with both hands.
The term "Retrospective Astonishment" (perhaps someone could translate this into Latin?) does capture the point. admiratio: admiration, wonder
retrospecio, retrospecere: to look back
retrospecatum: my guess at the participial form
My best guess at how it should look, assuming the ablative case follows in: Admiratio in retrospectato