To reproduce:
private void Form1_Load(object sender, EventArgs e)
{
    this.customersTableAdapter.Fill(this.nwindDataSet.Customers);
    this.radGridView1.DataSource = this.customersBindingSource;
    this.radGridView1.BestFitColumns(Telerik.WinControls.UI.BestFitColumnMode.AllCells);
    this.radGridView1.AllowSearchRow = true;
}

Workaround:
private void Form1_Load(object sender, EventArgs e)
{
    RadDragDropService svc = this.radGridView1.GridViewElement.GetService<RadDragDropService>();
    svc.PreviewDragDrop += svc_PreviewDragDrop;
}

private void svc_PreviewDragDrop(object sender, RadDropEventArgs e)
{
    GridViewSearchRowInfo.Cancel = true;
}