The used cell range is expanded when styles to all columns are applied and one copies an entire row.

Workaround:

private void RadSpreadsheet_WorkbookChanged(object sender, EventArgs e)
{
    var range = this.radSpreadsheet.Workbook.ActiveWorksheet.UsedCellRange;
    this.radSpreadsheet.ActiveWorksheet.Columns[range.ColumnCount + 1, SpreadsheetDefaultValues.ColumnCount - 1].Clear(ClearType.All);
}