I've searched Google high and low and can't find an answer.
Here's the series of events:
Cyrus Vance resigns on April 28, 1980
Warren Christopher in office from April 28 to May 3.
David Newsom in office on May 2nd and part of the day May 3rd.
Richard Cooper in office part of the day on May 3rd.
David Newsom back in office on May 3rd and leaves on May 4th.
Warren Christopher back in office from May 4th and leaves on May 8th.
Edmund Munskie sworn in as permanent Secretary of State on May 8th.
I can't figure out why it would go Vance, Christopher, Newsom, Cooper, Newsom, Christopher, and finally Munskie. I've searched and can't find any relevant political events around that time. I do know that Vance resigned out of protest.
Help!!!
Edits - formatting
Given the dates, you might imagine this had something to do with the Iranian hostage crisis, and you'd be correct. Vance resigned in protest of the planning and execution of the botched hostage rescue Operation Eagle Claw.
In particular, he had long-running issues with Zbigniew Brzezinski and Zbigniew's effort to execute the mission while Vance was away recuperating from gout was what fueled his decision to resign.
It's normal for a resigned cabinet position to be run by the undersecretary as the acting Secretary until the nomination of a permanent Secretary. I think your question is why did he run through so many acting Secretaries of State.
For this I don't have an answer, only to offer that Jimmy Carter's own Whitehouse diary didn't recognize the transition of acting Secretaries, unless we assume David Newsom and Mrs. Christopher were an item...
Meanwhile, the Office of the Historian in the Department of State does recognize that Warren Christopher held two separate interim titles only a day apart, separated by Newsom and Cooper.
Quite a puzzler. Still looking... I'll update if I find anything more.
The transcript of the May 3 meeting is confounding, as it switches between Secretary Christopher and Mr. Christopher seemingly at will.
A LexisNexis search shows no results in publication over May 4th to 8th for either Cooper or Newsom. No idea where to look from here, other than phone Jimmy Carter.
Sources:
https://history.state.gov/departmenthistory/people/principalofficers/secretary-ad-interim
https://history.state.gov/historicaldocuments/frus1977-80v28/d25
https://www.jimmycarterlibrary.gov/documents/diary/1980/d050380t.pdf
http://www.nytimes.com/2002/12/29/magazine/the-lives-they-lived-out-of-the-loop.html