Dropdown List issue in ASP Panel control - asp.net

Question

I am using asp Panel control in which I have a dropdown list for different Timezones to select.
However, When I select any timezone It disappears when I open the pop up next time.
Can you please suggest or provide some pointers ?
<asp:PlaceHolder ID="DisplayMode" runat="server">
<div id="ListBranch" style="text-align: left">
<asp:Button ID="AddBranch" runat="server" Text="Add Branch" OnClick="AddBranch_Click" />
<act:ModalPopupExtender ID="ModalPopupExtenderBranch" runat="server" TargetControlID="AddBranch" PopupControlID="pnlAddBranch"
CancelControlID="btnCancel" BackgroundCssClass="modalBackground">
</act:ModalPopupExtender>
<act:ToolkitScriptManager ID="smBranch" runat="server">
</act:ToolkitScriptManager>
<div>
<br />
<asp:GridView runat="server" ID="gvBranchList" DataKeyNames="BranchID" AutoGenerateColumns="false" Font-Size="Small">
<RowStyle BackColor="#EFF3FB" />
<FooterStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
<PagerStyle BackColor="#2461BF" ForeColor="White" HorizontalAlign="Center" />
<HeaderStyle BackColor="#336699" Font-Bold="True" ForeColor="White" />
<AlternatingRowStyle BackColor="White" />
<Columns>
<asp:BoundField DataField="Branch Name" HeaderText="Branch Name" />
<asp:BoundField DataField="Timezone" HeaderText="Timezone" />
<asp:TemplateField HeaderText="Edit">
<ItemTemplate>
<asp:Button ID="Edit" runat="server" Text="Edit" OnClick="Edit_Click" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
<br />
<asp:Label ID="lblresult" runat="server" />
<asp:Button ID="btnShowPopup" runat="server" Style="display: none" />
<asp:Button ID="UpdateBranch" CommandName="Update" runat="server" Text="Ok" OnClick="Ok_Click" Style="display: none" />
<asp:Panel ID="pnlAddBranch" runat="server" BackColor="White" Height="80%" Width="100%" Font-Size="Small" Style="top: 40px !important;">
<table style="border: solid 2px #336699; width: 100%; height: 100%; font-size: small;" cellpadding="0" cellspacing="0">
<tr style="background-color: #336699">
<td colspan="2" style="height: 5%; color: White; font-weight: bold; font-size: larger" align="center">Branch Details</td>
</tr>
<tr>
<td align="left">Branch Name: </td>
<td>
<asp:TextBox ID="BranchName" runat="server"></asp:TextBox>
</td>
</tr>
<tr>
<td align="left">Timezone: </td>
<td>
<asp:DropDownList ID="ddlTimezones" AutoPostBack="false" OnSelectedIndexChanged="ddlTimezones_SelectedIndexChanged" runat="server" />
</td>
</tr>
<tr>
<td align="left">Assigned Holidays: </td>
<td>
<asp:CheckBoxList ID="chkAssignedHolidays" runat="server" RepeatColumns="3" Font-Size="Small" />
</td>
</tr>
<tr>
<td></td>
<td>
<asp:Button ID="Ok" CommandName="Add" runat="server" Text="Ok" OnClick="Ok_Click" />
<asp:Button ID="Cancel" Commandname="Cancel" runat="server" Text="Cancel" OnClick="Cancel_Click" />
</td>
</tr>
<tr>
<td colspan="2">
<asp:Label ID="lblBranchID" runat="server" Style="display: none"></asp:Label>
</td>
</tr>
</table>
</asp:Panel>
</div>
</div>
</asp:PlaceHolder>