Adding loader: new Ext.tree.TreeLoader({ preloadChildren: true }) to the TreePanel config should work.
new Ext.tree.TreePanel({
renderTo: Ext.getBody(),
loader: new Ext.tree.TreeLoader({ preloadChildren: true }), //New Config
id: 'mainTree',
title: 'Packages',
useArrows: true,
autoScroll: true,
containerScroll: true,
frame: true,
rootVisible: false,
root: rootNode,
buttons: [traverse]
});