Try this:

        private void ultraGrid1_AfterCardCompressedStateChanged(object sender, AfterCardCompressedStateChangedEventArgs e)
        {
            // If a card is being collapsed, do nothing.
            if (e.NewCompressedState == CardCompressedState.Compressed)
                return;

            UltraGrid grid = (UltraGrid)sender;
            // Disabled this event so it does not fire recursively.
            // This is not strictly neccessary, since the code above will bail out when collapsing
            // a card, but it's more efficient for the event not to fire in the first place.
            grid.EventManager.SetEnabled(GridEventIds.AfterCardCompressedStateChanged, false);
            try
            {
                // Loop through all the rows.
                foreach (UltraGridRow row in e.Row.ParentCollection)
                {
                    // Don't affect the row being expanded.
                    if (row != e.Row)
                    {
                        // Collapse the card.
                        row.IsCardCompressed = true;
                    }
                }
            }
            finally
            {
                // Re-enabled the event
                grid.EventManager.SetEnabled(GridEventIds.AfterCardCompressedStateChanged, true);
            }
        }