I've found the support case that was raised in relation to this thread.  Here's a summary:

The behavior is expected because these changes to the data source caused focus to be put elsewhere in the grid.  The grid will scroll to bring the active cell/row into view.

As a solution, the above code for the AfterExitEditMode event handler was modified to save a reference to the originally-active row and to activate it again.

using Infragistics.Win.UltraWinGrid;
....
private void ultraGrid1_AfterExitEditMode(object sender, EventArgs e)
{
    Random r = new Random();
    ultraGrid1.ActiveRow.Cells["descrizione"].Value = "descrizione " + r.Next().ToString();

    UltraGridRow row = ultraGrid1.ActiveRow;
    UltraGridRow getRow = ultraGrid1.ActiveRow;

    while (row.ParentRow != null)
    {
        row = row.ParentRow;
        DataRow dr = ((DataRowView)row.ListObject).Row;

        if (dr != null)
            dr["descrizione"] = "descrizione " + r.Next().ToString();
    }

    getRow.Activate();
}