DIV content shows on page instead of JQuery Dialog

I have the following DIV markup:

<div id="dialog" title="Membership Renewal">
Your membership is going to expire.
</div>

I have the following javascript to execute the JQuery:

<script type="text/javascript">
function showjQueryDialog() {
    $("#dialog").dialog("open");
    //alert("Time to renew Membership!");
}

$(document).ready(function() {
    $("#dialog").dialog({
        autoOpen: false,
        modal: true,
        buttons: { "Renew Membership": function() { $(this).dialog("close"); } }
    });
});
</script>

I have an asp:Button which is inside a control and the control is on a master page. The first thing I notice is that when the page is loaded, the div is displayed and then disappears when the page is done loading. When I click the button it executes the following:

if (timeSpan.Days >= 30)
{
//Show JQuery Dialog Here
ScriptManager.RegisterClientScriptBlock(this, typeof(Page), "showExpiration",    
"showjQueryDialog()", true);
}

When I click the button, instead of a dialog popping up, the content of the div just becomes visible.