The proceeding three perceptions (see Forsyth Criteria) (that, what, who) are straightforward. "Why" is much different. Dave Snowden's comment that sense is made retrospectively--after the fact--seems to be true. While it might be nice to know WHY someone went to the trouble to make a change, I will still be affected by the change and something interesting may happen to me.
I wonder if there may be a starter list of answers to the question: Why would you change another's writing?
Misspelling
Improvement in grammar.
Correction of factual error.
Extension of the concept.
Reframing.
Adding context.
Definition adding.
Poetic ramification stimulated by the prior piece.
Opposition to prior piece.
Changing style--exposition, narration, video, visual art, etc.