The ItemsRequested event is fired when a callback is initiated from the client-side - when the user types in the combobox input field or when the user clicks the drop-arrow image. To fire the ItemsRequested event, you should first enable the EnableLoadOnDemand property of the RadComboBox object.
Since the ItemsRequested event is fired when a callback is initiated form the client-side, all controls that are being updated in the ItemsRequested event handler will lose the changes after the callback. This behavior is due to the async (callback) request that is made to fire the ItemsRequested event.
A working solution would be wrapping the combobox and the controls that will be updated with an AJAX Panel. You should, however, keep into account that you should not use the built-in callback machinism of Telerik RadComboBox along with Telerik RadAjax.You should entirely use the combobox in a server mode - adding the items in the Load event of the page for instance. The AJAX Panel will take care of transforming the postback into a callback and thus keeping the needed client-side experience.