We select a RadTreeViewItem by setting its property IsSelected to "True".
As a result, first "SelectionChanged" event is fired and second  - "Selected" event is fired.
If the selection is made "normally", by selecting an Item (with the keyboard or the mouse), the above events are fired in reversed order - first the "Selected" event and second the "SelectionChanged" evnet.

The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.