In your code you create select element with options having values equal to data-filter attribute of your anchor elements
jQuery( "#filters a" ).each( function() {
var el = jQuery( this );
jQuery( "<option />", {
"value" : el.attr( "data-filter" ),
"text" : el.text()
}).appendTo( "#filters select" );
});
then you bind onchange event handler to your select element, so that it redirects you to the page with address equal to the option value (i.e. your anchor's data-filter value)
jQuery( "#filters select" ).change( function() {
window.location = jQuery( this ).find( "option:selected" ).val();
});
so, it works as it is supposed to - redirecs you to a page with the "data-filter"
I think in your case you don't need that .change handler at all to let isotope do its job