To reproduce:

Setup RadGridView as follows:

this.Grid.MasterTemplate.EnablePaging = true;
this.Grid.Columns.Add("");
this.Grid.Columns.Add("");
this.Grid.Columns.Add("");
this.Grid.Columns.Add("");

On a button click add rows:

private void Button_Clic9k(object sender, EventArgs e)
{
    for (int i = 0; i < 120; i++)
    {
        this.Grid.Rows.AddNew();
    }
}

You will see a vertical scrollbar when it is not needed.

Workaround:

Wrap the loop in a Begin/EndUpdate calls:

private void Button_Clic9k(object sender, EventArgs e)
{
    this.Grid.BeginUpdate();
    for (int i = 0; i < 120; i++)
    {
        this.Grid.Rows.AddNew();
    }

    this.Grid.EndUpdate();
}