Thank you for contacting us.
I am afraid you cannot avoid calling of NeedDataSource
event handler when you expand or collapse an item. The reason is that the whole data could not be initially loaded and when expanding an item the needed data is retrieved from the datasource. Nevertheless if you want initially to display RadTreeList empty then you can add the isPostBack check in NeedDataSource event.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.