Page_Load called twice

Hello,
could someone explain to me why following page calls the Page_Load event twice when the user clicks on the delete button?

 test.aspx:
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="test.aspx.vb" Inherits="test" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Unbenannte Seite</title>
</head>
<body>
<form id="form1" runat= ...