You are adding the class dynamically after the first drggable is fired on your elements and will not live evalueted.
You can add the drggable when you are adding the class and enable/disable it if it's already attached:
$("article.spread").dblclick(function () {
$(this).toggleClass("draggable");
if ($(this).data('draggable')) {
$(this).draggable('option', 'disabled') ? $(this).draggable('enable') : $(this).draggable('disable');
} else {
$(this).draggable();
}
});