The current implementation of RadTreeView's data layer requires that all root level nodes have the same ParentId. The architecture requires that root level nodes are ordered by their ParentID. In the case where nodes with negative ParentIds are added they go to the top of the root nodes collection. Given this and the requirement to have only one node as root, the last added node becomes the root node.
If you have a different data structure you can use the Load On Demand feature of RadTreeView http://www.telerik.com/help/winforms/treeview-data-binding-load-on-demand.html