Hi,
Maybe you have TreeViewPopulateOnDemand enabled or something, or maybe it has changed since version 2.2.8.1, but if you look at this site, the developer docs page, you will notice it can expand and collapse treeview nodes with javascript not using postback.
If you are creating some custom menu using the ASP.NET treeview then maybe use an UpdatePanel wrapped around it to do ajax postbacks.
Hope it helps,
Joe