We investigated this behavior and it seems that the logic in the Selector class (which the RadTabControl inherits) that raises the SelectionChanged event listens if the tab control has an tab control parent. If so it fires the parent's event.
In order to change this behavior you can subscribe for the SelectionChanged event of the inner RadTabControl and handle the event.
sender, RadSelectionChangedEventArgs e)
I hope this helps.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.