Why NSApplicationDelegate method openFiles: is being called multiple times on a multiple drag to the dock icon?

I've seen this behavior in one of my apps (usually when dragging a whole bunch of files at one time). As I workaround, instead of opening the files directly from application:openFiles:, I queue them up and open the queued files after a small delay. Something like the following:

- (void) application:(NSApplication*)sender openFiles:(NSArray*)filenames{    // I saw cases in which dragging a bunch of files onto the app    // actually called application:openFiles several times, resulting    // in more than one window, with the dragged files split amongst them.    // This is lame.  So we queue them up and open them all at once later.    [self queueFilesForOpening:filenames];    [NSApp replyToOpenOrPrint:NSApplicationDelegateReplySuccess];}- (void) queueFilesForOpening:(NSArray*)filenames{    [self.filesToOpen addObjectsFromArray:filenames];    [self performSelector:@selector(openQueuedFiles) withObject:nil afterDelay:0.25];}- (void) openQueuedFiles{    if( self.filesToOpen.count == 0 ) return;    [self makeNewWindowWithFiles:self.filesToOpen];    [self.filesToOpen removeAllObjects];}