[~alex.parvulescu] noted that OakDirectory saves the directory listing even if no actual change happened in the directory. Only change that happens is the order of entries in set.
In normal cases LuceneIndexEditor avoids initializing the IndexWriter if there is no change. However it can happen that when any node gets deleted the editor performs a delete operation. It can happen that tree being deleted is not indexed but still editor would do this as it cannot determine that easily. This would lead to OakDirectory being closed without any change and thus can lead save of dir listing with just change in order of entries