If you drop the TreeNode on an element that has no ID, the TreeView does not postback automatically. This is because there is no way to communicate to the server where was the node dropped and to prevent postbacks if the user accidentally drops the node on some unrelated element.
For your scenario to work, you need to use the OnClientNodeDropping event and set the htmlElement manually to an element that has an ID. This is very similar to the approach used in the Drag and Drop demo
of the TreeView and more specifically the droppedOnGrid function that determines if the target element is part of the Grid and then sets the Grid as the 'target' element using args.set_htmlElement:
var target = args.get_htmlElement();
if(target.id == gridId)
target = target.parentNode;
I hope this helps.
the Telerik team