I have a feeling I know what may have happened.
The data errors project is making it really easy to make a LOT of contributions, easily, to a large number of profiles in a short amount of time.
One of the errors is data-less/dateless profiles. Error 902 if you're interested. (I requested this one.)
It sounds like the person in question might have been working on 902, or one of the other profiles that show up on one of the other error lists. Came across an open profile with no date; added an estimated date, and if it was under 100 years ago, the profile became private in the process. So it wasn't private when s/he made the edit, but became so after.
That said s/he should have checked for active profile manager status first before making changes.