If a tree node is selected, then another node is selected, then the first node is re-selected, the first node will not fire an IsSelected event the second time it was selected.  The event is potentially fired when the node provider receives an AfterSelect event.  The event then decides whether to fire by looking at its cached old property value and comparing it with the new value.  Since AfterSelect is only ever called after the node has been selected, the new value will always be true, so the old value will become true and will never be reset to false when the node is deselected, so, on subsequent calls, the BaseAutomationPropertyEvent code will not fire the event since it thinks that the node was already selected.