To reproduce: use the following code snippet and perform the steps illustrated on the attached gif file.

radGridView1.Columns.Add(new GridViewTextBoxColumn("A", "A"));
radGridView1.Columns.Add(new GridViewTextBoxColumn("B", "B"));
radGridView1.Columns.Add(new GridViewTextBoxColumn("C", "C"));
radGridView1.Columns.Add(new GridViewTextBoxColumn("D", "D"));
radGridView1.Columns.Add(new GridViewTextBoxColumn("E", "E"));
radGridView1.Columns.Add(new GridViewTextBoxColumn("F", "F"));


radGridView1.Columns[0].Width = 150;
radGridView1.Columns[1].Width = 150;
radGridView1.Columns[2].Width = 150;
radGridView1.Columns[3].Width = 150;
radGridView1.Columns[4].Width = 150;
radGridView1.Columns[5].Width = 150;

radGridView1.Rows.Add("A", "B", "C", "D", "E", "F");
radGridView1.Rows.Add("A", "B", "C", "D", "E", "F");
radGridView1.Rows.Add("A", "B", "C", "D", "E", "F");
radGridView1.Rows.Add("A", "B", "C", "D", "E", "F");
radGridView1.Rows.Add("A", "B", "C", "D", "E", "F");
radGridView1.Rows.Add("A", "B", "C", "D", "E", "F");
radGridView1.Rows.Add("A", "B", "C", "D", "E", "F");

Workaround: 

private void radGridView1_Resize(object sender, EventArgs e)
{
    if (this.radGridView1.IsInEditMode)
    {
        this.radGridView1.EndEdit();
        this.radGridView1.BeginEdit();              
    }
}

public class CustomGrid : RadGridView
{
    public override string ThemeClassName  
    { 
        get 
        { 
            return typeof(RadGridView).FullName;  
        }
    }

    protected override RadGridViewElement CreateGridViewElement()
    {
        return new CustomRadGridViewElement();
    }
}

public class CustomRadGridViewElement : RadGridViewElement
{
    protected override Type ThemeEffectiveType     
    { 
        get    
        { 
            return typeof(RadGridViewElement);     
        }
    }

    protected override GridViewEditManager CreateEditorManager()
    {
        return new CustomGridViewEditManager(this);
    }
}

public class CustomGridViewEditManager : GridViewEditManager
{
    public CustomGridViewEditManager(RadGridViewElement gridViewElement) : base(gridViewElement)
    {
    }

    protected override void InitializeEditor(IInputEditor activeEditor)
    {
        if (activeEditor == null)
        {
            activeEditor = this.GridViewElement.CurrentColumn.GetDefaultEditor();
            this.ActiveEditor = activeEditor;
        }
        base.InitializeEditor(activeEditor);
    }
}