Web Forms :: Master And Base Pages For Default.aspx - Page_Load Is Called Twice?

I've got a page Default.aspx, it was inherited from class MyBasePage

like this

public partial class _Default : MyBasePage, ICallbackEventHandler
{
}

also it has a master page

<%@ Page Language="C#" AutoEventWireup="true" EnableEventValidation="false" MasterPageFile="~/Master/MyMaster.master" CodeFile="Default.aspx.cs" Inherits="_Default" %>

this master page also has a master

<%@ Master Language="C#" MasterPageFile="~/Master/Base.master" AutoEventWireup="true" CodeFile="MyMaster.cs" Inherits="MyMaster" %>

All works ok, but then i discovered that

Page_Load in Default.aspx is called two times, and both from method OnLoad in MyBasePage

protected override void OnLoad(EventArgs e)
{
base.OnLoad(e);
}
base.OnLoad(e) calls Page_Load.

I've got a site with such a structure of master pages and basepage, but it works ok, method page_load is called only once.

So why would in the second site PAge_Load be called twice?