I got it working OK. The event fires properly and the event handler is called. The only thing I can spot is that you are using both RadAjaxManger and RadAjaxPanel in one and the same page, which is not supported, and may lead to broken AJAX on the page. Use one only. Additionally, if you use RadAjaxManager, it needs to go in the master page, where the content page should have a RadAjaxManagerProxy, as described here
Other than that, I can see that your page does not postback at all when you miss to fill the complete form, due to the large number of required field validator controls.
Shortly, I removed the field validators and set up AJAX properly. The SelectedIndexChanged
event works OK.
the Telerik team