How can one see why and when a particular tag was deleted and who did decided upon this?
That tag wasn't deleted. All questions that were tagged, had the tag removed. The tag will be deleted soon, though - the Great Orphan Reaper strikes once a day.
I'm not aware of any way to discover the questions that most recently had the tag, other than the really hard, one-question-at-a-time way. You might get lucky if Google/DDG/Bing hasn't indexed the question since the tag was removed, or you might search for questions that would have had the tag at one point.