Put your model pop in Update panel and use Trigger for the drop down selected index change event. will solve your problem,
Example
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel2" runat="server" CssClass="pnlAlign">
<ContentTemplate>
<asp:Button runat="server" ID="btnShowModalPopup" Style="display: none" />
<ajax:ModalPopupExtender ID="mpopRejectAndRework" runat="server" TargetControlID="btnShowModalPopup"
PopupControlID="divPopUp" BackgroundCssClass="modalBackground" DropShadow="true"
RepositionMode="RepositionOnWindowResize" />
</ContentTemplate>
<asp:DropDownList ID="ddlStatus" runat="server" Width="200px" AutoPostBack="True" CssClass="input"
OnSelectedIndexChanged="ddlStatus_SelectedIndexChanged">
</asp:DropDownList>
<Triggers>
<asp:PostBackTrigger ControlID="ddlStatus" />
</Triggers>
</asp:UpdatePanel>