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?
Improvement in grammar.
Correction of factual error.
Extension of the concept.
Poetic ramification stimulated by the prior piece.
Opposition to prior piece.
Changing style--exposition, narration, video, visual art, etc.