Why the ContentDialog.ShowAsync() flickers?

In wp 8.1, I used ContentDialog.ShowAsync() to switch from current ContentDialog to another, and it flickers. It is like this, there are two ContentDialog A and B, when I try to switch from A to B, B appears and disappears, and A appears and disappears, then B appear and keep shown up. I don't know why A will appear? Any help?

            if (_contentDialog != null)
            {
                _contentDialog.Content = null;
                _contentDialog.Opened -= OnContentDialogOpened;
                _contentDialog.Closed -= OnContentDialogClosed;
            }

            _contentDialog = new ContentDialog();
            _contentDialog.Opened += OnContentDialogOpened;
            _contentDialog.Closed += OnContentDialogClosed;
            _contentDialog.FullSizeDesired = true;


            //_BMessageDialogRoot.Opacity = 1;
            _BMessageDialogRoot.Margin = new Thickness(0, StatusBar.GetForCurrentView().OccludedRect.Height, 0, 0);
            _contentDialog.Content = _BMessageDialogRoot;
            await Task.Delay(100);

            await _contentDialog.ShowAsync();