I've stripped this example down in hopes an answer might be easier to come by.

• I have a simple editable WHDG with nothing defined in the .aspx
  <igtbl:WebHierarchicalDataGrid id="MyGrid" runat="server" CssClass="WebGrid" AutoGenerateColumns="false" EnableDataViewState="true" EnableViewState="true">
• At runtime define two columns, UnitId and Status. UnitId column will use the standard text editor and Status column will use a infragistics dropdowneditor.

    BoundDataField boundDataField = new BoundDataField(true);
    boundDataField.DataFieldName = "UnitId";
    boundDataField.Key = "UnitId";
    boundDataField.Header.Text = "";
    boundDataField.DataType = ColumnDataType.Int;
    boundDataField.Width = new Unit(150);
    boundDataField.Hidden = false;
    boundDataField.DataFormatString = "";

    if (MyGrid.GridView.Columns[boundDataField.Key] == null) {
      MyGrid.GridView.Columns.Add(boundDataField);
    }

    BoundDataField boundDataField2 = new BoundDataField(true);
    boundDataField2.DataFieldName = "Status";
    boundDataField2.Key = "Status";
    boundDataField2.Header.Text = "Status";
    boundDataField2.DataType = ColumnDataType.String;
    boundDataField2.Width = new Unit(150);
    boundDataField2.Hidden = false;
    boundDataField2.DataFormatString = "";

    if (MyGrid.GridView.Columns[boundDataField2.Key] == null) {
      MyGrid.GridView.Columns.Add(boundDataField2);
    }

• This seems to work fine. Both my columns appear in the grid on pageload and across postbacks.
• Next I attempt to add editing behaviors to make the grid editable.

//ACTIVATION
    MyGrid.Behaviors.CreateBehavior<Infragistics.Web.UI.GridControls.Activation>();
    MyGrid.Behaviors.Activation.Enabled = true;

//EDITING CORE
    MyGrid.Behaviors.CreateBehavior<Infragistics.Web.UI.GridControls.EditingCore>();
    MyGrid.Behaviors.EditingCore.Enabled = true;
    MyGrid.Behaviors.EditingCore.AutoCRUD = false;
    MyGrid.Behaviors.EditingCore.BatchUpdating = true;
    MyGrid.Behaviors.EditingCore.EnableInheritance = true;

//CELL EDITING
    MyGrid.Behaviors.EditingCore.Behaviors.CreateBehavior<Infragistics.Web.UI.GridControls.CellEditing>();
    MyGrid.Behaviors.EditingCore.Behaviors.CellEditing.Enabled = true;
    MyGrid.Behaviors.EditingCore.Behaviors.CellEditing.EditModeActions.EnableOnActive = true;
    MyGrid.Behaviors.EditingCore.Behaviors.CellEditing.EditModeActions.EnableOnKeyPress = true;
    MyGrid.Behaviors.EditingCore.Behaviors.CellEditing.EditModeActions.MouseClick = EditMouseClickAction.Single;

• This works okay on initial page load, but across postbacks the dropdown editor goes haywire. Below is the remaining code where I wire things together

//DEFINE DROPDOWN ITEMS
    List<DIYDropDownItem> items = new List<DIYDropDownItem>();
    items.Add(new DIYDropDownItem("Status1", "A", false));
    items.Add(new DIYDropDownItem("Status2", "M", false));
    items.Add(new DIYDropDownItem("Status3", "D", false));
    items.Add(new DIYDropDownItem("Status4", "I", false));

    List<DropDownItem> ddi = new List<DropDownItem>();

    foreach (DIYDropDownItem item in items) {
      ddi.Add(CreateDropDownItem(item.Text, item.Value, item.Selected));
    }

//DEFINE DROPDOWN PROVIDER
    DropDownProvider dropDown = new DropDownProvider();
    dropDown.ID = "DropDownProvider1";
    dropDown.EditorControl.DisplayMode = Infragistics.Web.UI.ListControls.DropDownDisplayMode.DropDownList;
    dropDown.EditorControl.TextField = "Status";
    dropDown.EditorControl.ViewStateMode = System.Web.UI.ViewStateMode.Enabled;
    dropDown.EditorControl.EnableViewState = true;

    foreach (DropDownItem ddi2 in ddi) {
      dropDown.EditorControl.Items.Add(ddi2);
    }

//ASSIGN DROPDOWN PROVIDER TO GRID'S EDITORPROVIDERS COLLECTION
    MyGrid.EditorProviders.Add(dropDown);

//LINK EDITOR TO A COULMN VIA COLUMN SETTING
    EditingColumnSetting setting = new EditingColumnSetting();
    setting.ColumnKey="Status";
    setting.EditorID = "DropDownProvider1";
 
//ADD SETTING TO GRID
    MyGrid.Behaviors.EditingCore.Behaviors.CellEditing.ColumnSettings.Add(setting);

//UPDATE BEHAVIORS TO ENSURE THEY ARE REFLECTED IN UI
    MyGrid.RefreshBehaviors();