There are large number of items for the TreeList (for example 3000) and you have all them initially collapsed.
For example you could modify the IsExpandedBinding demo to have 3000 items.
Then scroll down so the row with Drinks and Count = 2194 is at the top of the control and expand it.
The result is that both that row and the row with Drinks and Count = 2199 expand.

Reproducible on http://demos.telerik.com/silverlight/#TreeListView/TreeListViewDragAndDrop

Asynchronous mode for ExpandAllHierarchyItems()

Items are expanded when they match the filtering criteria.

Once I added the filterdescriptor, the tree started to expand itself
and when I put a break point in RowIsExpandedChanged event handler
the treelistviewrow's IsExpanded property is already true

Can you have a look at the attached sample application
and try to achieve the following:
1. keep the on demand load (so only loadchildren when user clicked on the arrow to expand)
2. apply filter only on the expanded nodes

TreeListView is trying to expand its item which is empty.