HTML Why is the content of an Iframe loaded twice?

Question

In my application I use a javascript function to set the src tag of an Iframe:

function loadDocument(id, doc) {
    $("#DocumentContent").show();
    $("#ButtonBox").show();

    // Clear dynamic menu items
    $("#DynamicMenuContent").html("");
    $("#PageContent").html("");

    // Load document in frame
    $("#iframeDocument").attr("src", 'ViewDoc.aspx?id=' + id + '&doc=' + doc + '');

    // Load menu items
    $("#DynamicMenuContent").load("ShowButtons.aspx");

    // Set document title
    $("#documentTitle").load("GetDocumentInfo.aspx?p=title");
}

When I open Fiddler and debug this page, I notice that the 'ViewDoc.aspx' page is called twice.

When I put an alert() in the loadDocument function, I get only one alert message. In my viewdoc.aspx page there are no refresh or redirect statements or other statements that refreshes the page.

Is it possible this has something to do with browser? Is this default browser behavior?