The TreeViewItems are expanded and collapsed on MouseLeftButtonUp which leads to an expand/collapse operation even if the MouseLeftButtonDown event was fired by another element. This can be easility reproduced if you hold down the mouse left button over any element outside the RadTreeView and then releasing it over any RadTreeViewItem when the RadTreeView IsExpandOnSingleClickEnabled is True.

It can also be reprorduced with the following steps:
1) left click on a node and hold the mouse button down
2) release the mouse when over a non-selected node that has children.
The result is expanding/collapsing the unselected node.