SelectedIndexChanged now triger with dynamic dropdown

I am creating multi-level categories under dropdown... 
My first drop down is fixed in aspx file and as per the login i need to create other drop downs on selection of parent drop down 

Here is ASPX code 
 <asp:DropDownList ID="categoryDropDown" runat="server" CssClass="form-control" AutoPostBack="True" OnSelectedIndexChanged="categoryDropDown_SelectedIndexChanged"></asp:DropDownList>
<asp:Panel ID="DynamicDropDownPanel" runat="server"></asp:Panel>

Here is Code behind
// First Drop Down It's working fine   protected void categoryDropDown_SelectedIndexChanged(object sender, EventArgs e)
{ DynamicDropDownPanel.Controls.Add(generateDynamicDropDown(categoryDropDown.SelectedValue ...