I have just discovered the source of the problem. I have been using the FormDecorator on most of my pages. When I remove the decorator control below from the problem page the problem described in this thread goes away. So it seems there is a clash of some sort between the decorator and the checkboxes on the treeview control under particular conditions.
telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="CheckBoxes" />