Why NSManagedObjectContextDidSaveNotification sent on background thread?

Question

My app has a single MOC that is created on the main thread when app is launched. However, I get unexpected behavior (very rarely) where NSManagedObjectContextDidSaveNotification is posted on a background thread while I expect it on the main thread. Here is the code:

  [[NSNotificationCenter defaultCenter] addObserverForName:NSManagedObjectContextDidSaveNotification
                                                      object:self.managedObjectContext
                                                       queue:nil
                                                  usingBlock:^(NSNotification *notification) {
       if (![NSThread isMainThread]) {
          // Rarely getting here, not sure why?
       }
   
  }];

The self.managedObjectContext refers to the MOC instance I create on the main thread when app is loaded.

Any ideas what could be the reason?