asynchronous postback interfering with button click events

Sys.WebForms.PageRequestManager.getInstance().add_beginRequest is causing a button click event not to fire.

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                string enableOnTimeoutScript ="<script type='text/javascript'>Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(" +"function (sender, args) { if (args.get_error()) { ";

                int buttonIndex = 0;

                foreach (Button button in allButtons)
                {
                    enableOnTimeoutScript +=
                        string.Format("var button{0} = document.getElementById('{1}'); " +"if (button{0} != null) button{0}.disabled = true; ", buttonIndex++, button.ClientID);
                }

                enableOnTimeoutScript +=
                    string.Format("var validator0 = document.getElementById('{0}'); " +"if (validator0 != null) {{ validator0.style.visibility = 'visible'; " +"validator0.innerHTML = args.get_error().message; }} ", customValidator.ClientID);

                enableOnTimeoutScript += "args.set_errorHandled(true); } });</script>";

                Literal enableOnTimeoutLiteral = new Literal();
                enableOnTimeoutLiteral.Text = enableOnTimeoutScript;

                panelDocumentNav.Controls.Add(enableOnTimeoutLiteral);
            }
                
            Button saveData = new Button();
            saveData.EnableViewState = true;
            saveData.Click += new EventHandler(dynamicButton_Click);
            saveData.Text = "SAVE";
      }

 <asp:Content ID="contentBody" ContentPlaceHolderID="contentPlaceHolderBody" runat="server"><div style="height: 100%; left: 0px; position: absolute; top: 0px; width: 76%;"></div><asp:Panel ID="panelDocumentNav" runat="server" CssClass="panel" Width="24%" Style="height: 100%;
        position: absolute; right: 0px; top: 0px;"><div class="spacer" style="overflow: auto; height: 100%"><asp:PlaceHolder runat="server" ID="dynamicControls"></asp:PlaceHolder></div><asp:UpdatePanel ID="updatePanelQ" runat="server"><ContentTemplate><div><asp:CustomValidator ID="customValidator" runat="server" CssClass="error" /></div></ContentTemplate></asp:UpdatePanel>

When the page is first loaded and the SAVE button is clicked the dynamicButton_Click event is not fired but a post back occurs b/c the Page_Load method is called but the if (!IsPostBack) is false so code within if statement is not executed.  The page is reloaded.  If I click the button again the dynamicButtonClick event is fired.

If I comment out the code in the if (!IsPostBack) the dynamicButton_Click event is fired on the first click.

Why is the event not being fired on the first click?