I believe this question was closed as a dup (I voted to close it).
Who reopened it? Why?
It was never closed. Clicking the "timeline" link on the question shows its entire status history: this is available to all users regardless of rep.
Here is the timeline of the question you linked: nowhere in the history does it show closure. For reference, here is the timeline of a question that actually was closed. Specifically you're looking for changes to the "state" of the question, like this:
Your linked question has no such state changes in its timeline.
Also, simply voting to close does not appear on the timeline (or history) - only actual status changes that are completed (by community vote or mod vote, for example). I believe closure votes expire after a time if closure is not completed, though I'm not 100% sure of exactly how much time must pass.