To reproduce:

public Form1()
{
    InitializeComponent();
    this.radGridView1.Columns.Add("Name");
    this.radGridView1.Columns.Add("ID");

    this.radGridView1.RowsChanged += radGridView1_RowsChanged;
}

private void radGridView1_RowsChanged(object sender, GridViewCollectionChangedEventArgs e)
{
    Console.WriteLine(e.Action);
    if (e.Action == Telerik.WinControls.Data.NotifyCollectionChangedAction.Add)
    {
        GridViewRowInfo row = e.NewItems[0] as GridViewRowInfo;
        row.Cells["ID"].Value = this.radGridView1.Rows.Count;
    }
}

private void radButton1_Click(object sender, EventArgs e)
{
    this.radGridView1.Rows.Add("Item" + this.radGridView1.Rows.Count);
}

Note: the first firing of the RowsChanged event used to be with Action=Add.