A distinguishing feature of the tectogrammatical analyses, vis a vis the other treebanks we are using, is that semantically empty words only take part in the analytical annotation layer and nodes are inserted at the tectogrammatical layer to represent covert elements of the sentence not present in the surface syntax of the analytical layer.
a pre-order traversal would result in tokens close to the root having few options, and in particular if the root has a single child, that node has no possible new heads unless one of its children has been assigned the root as its new head first.