Button in UpdatePanel not firing Click event: RequiredFieldValidator the cause - asp.net

Question

I'm working on a page that consists of Telerik RadToolTip, an UpdatePanel that contains a Button, and a few user controls. There is also a Master Page, which is where the ScriptManager is located.
Previously, I had a RadAjaxManager on the Master Page and RadAjaxProxyManager. I removed them and began to implement standard Update Panels because of complexities within the User Controls.
Now, however, I can't seem to get any of this page to behave as expected. When I place a Button in the UpdatePanel, I expect a Partial Postback and I expect the Button's OnClick event to be fired. However, when I run this code with a breakpoint in the btnPostBack_Click event code, the breakpoint never gets hit.
If I cut and paste the same controls with the same settings into a new project, the breakpoint gets hit. Is there something in the environment of the site or could Telerik be interfering with the Button's Click event? I've removed all RadAjaxManager/Proxies.
<%# Page Title="" Language="C#" MasterPageFile="~/AV.Master" AutoEventWireup="true"
CodeBehind="AssetView.aspx.cs" Inherits="AV_ASP_UI.AssetPages.AssetView" %>
<%# Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>
<%# Register TagPrefix="uc" TagName="AssetDiagram" Src="~/Controls/AssetDiagram.ascx" %>
<%# Register TagPrefix="uc" TagName="AssetInfo" Src="~/Controls/AssetInfo.ascx" %>
<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
<script type="text/javascript">
function DoPartialPostBack(command, id) {
document.getElementById("<%= hfldId.ClientID %>").value = id;
document.getElementById("<%= btnPostBack.ClientID %>").click();
}
</script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<asp:HiddenField ID="hfldId" runat="server" />
<telerik:RadToolTip ID="RadToolTip1" runat="server" Modal="true" ShowEvent="FromCode"
Position="Center" RelativeTo="BrowserWindow" HideEvent="FromCode" TargetControlID="hfldFireToolTip">
</telerik:RadToolTip>
<asp:UpdatePanel ID="UpdatePanel1" runat="server" >
<ContentTemplate>
<asp:Button ID="btnPostBack" runat="server" Text="Button" style="visibility: hidden" OnClick="btnPostBack_Click"/>
</ContentTemplate>
</asp:UpdatePanel>
<asp:Panel ID="pnlWorkArea" CssClass="pnlWorkArea" runat="server" Height="770px" onMouseDown="hideSVGTtip(event);">
<uc:AssetDiagram ID="ucAssetDiagram" runat="server" CanvasHeight="720" CanvasWidth="276" />
<uc:AssetInfo ID="ucAssetInfo" runat="server"/>
</asp:Panel>
</asp:Content>