To answer your direct question: A review item gets marked as not reviewable when the conditions for its creation no longer apply. For example, if a post gets deleted when it was in a review queue, the review item is not destroyed, it's just no longer reviewable (however, not complete). Or if a question gets closed outside of the Close Votes review queue, review does not complete, but the item becomes unreviewable.
Review items that are no longer reviewable are not supposed to get served to users. I have no idea why you would have been shown that one. It could be possible that a user Skipping a review audit doesn't cause it to become immediately unreviewable and you were just unlucky and attempted to load one in that tiny timeframe.
Update: It's now been reviewed, so it could have been a race condition for the audit. A dev will have to explain what exactly happens with review audits.