The prevailing wisdom in the bike industry during the days of steel frames was if you have the telltale ripples on the top-and down-tubes, the bike was in an accident and the failure is not the result of a manufacturing defect. If there is no ripple on the underside of the top-tube then it possibly let go because of a manufacturing defect.
My guess is that the head tube was separated from the top tube before the accident, or as a result of a less serious hit (went through a sewer grate turned the wrong way, for instance), and when the top tube and head tube was completely separated, the down tube held on for a bit, and was deformed before it let go completely.