A non-official answer ... It happens when the moderators decide that the title does not accurately reflect the content, or that it is otherwise somehow better

A non-official answer ... It happens when the moderators decide that the title does not accurately reflect the content, or that it is otherwise somehow better served by being something different. Sometimes the change is contentious, sometimes the change is undeniably better. There is also a short (but variable) window during which the submitter can edit a title, but that's unlikely to be the case in this instance, because to the best of my knowledge the window is at most 2 hours.