Button click triggers fullpage postback on first click

Despite the fact that I have placed my asp:Button inside an UpdatePanel it is still triggering a postback on the fullpage the first time it's clicked. Also the OnClick event isn't being caught the first time I click the button either, but every single time after that everything works fine. Any ideas what could be causing this problem? See code below;
(In my Site.Master file)
<asp:ScriptManager runat="server" AjaxFrameworkMode="Enabled" EnablePartialRendering="true" ValidateRequestMode="Disabled"> </asp:ScriptManager> (In my actual webpage)
<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="Editor.aspx.cs" Inherits="Technology.WebForm1" validateRequest="false" %> <asp:Content ID="Content1" ContentPl ...