What you observe is the expected behavior. A UI element
that is not visible (e.g. the items of a RadPanelBarItem that is collapsed) will not be initialized and any bindings regarding that element will not occur until the element needs to be rendered. The above is true for the Silverlight platform as a whole and is not specific to RadControls for Silverlight.
I hope I was able to help. Let me know if you have other questions.
the Telerik team