From BlenderWiki
When you drop a node with at least one input and one output socket onto a an existing connection between two nodes, auto-offset will, depending on the direction setting, automatically move the left or right node away to make room for the new node. It is triggered by the same logic that is used to automatically connect the new node, indicated by the highlighted noodle. You can toggle the direction offset while you are moving the node by pressing „T“. The auto-offset is enabled by default but can be disabled in the header of the node-editor. The offset margin can be changed in the editing section of the User Preferences.
