This question was reopened by a mod recently; it had been closed as duplicate for 3 months. I can see no changes in the question nor the question linked as duplicate, so what warranted the reopening?
I'm not the mod in question, but let me take a guess: because the answer quality is perceived as higher.
That's an interesting statement, given that the answer on the original dupe target has 6 votes, and the answer on the reopened post has 3. It's even more thorny given that it's an answer by the mod that reopened the question. To clarify a little:
When I read the top answer on the dup target, and then read the answer on the now-reopened post, I felt that the answer on the latter was better - before seeing who wrote it.
In some case where that's true, it can be worth reversing the duplicate, so that the better answer is on the open question. The duplicate hasn't been reversed in this situation, so I'll leave it to the mod in question to shed light on that.
Personally, given that the answer is by the reopening mod, I wouldn't gone anywhere near reopening it, for precisely this reason of the appearance of impropriety. However, there may well be reasons I haven't picked up on, so let it be down to the mod in question to explain.
I've been meaning to write an answer to this for a while, but I haven't really been able to since I can't load Stack Exchange without HTTPS, and Meta's can't load over HTTPS (For the record, I'm writing this answer from school).
It's been a while, so I don't exactly remember what I was thinking, but I vaguely remember cleaning up many questions, making sure that questions were properly tagged, especially with the licensing tag. I read almost all that I see, questions and answers, and I don't generally pay attention to other things: who wrote it, when the answer was made or other data, unless I'm specifically looking for it. I probably read the question, thought it wasn't a duplicate, and reopened it.
That being said, I kind of find the 'mod' part kind of... irrelevant and unfair. I may have a binding vote, which I generally forget that I have most of the time, but the fact that an emphasis was placed on the fact that I have a diamond doesn't place the point for me. As ArtOfCode said in a comment to this, I'm generally always available in chat, and I'm perfectly available to discuss these sorts of issues whenever I'm around. What I also find unfair, is that you didn't try anything else to reverse the action either: You could've cast a close vote as well if you disagreed with it.